I have a deployment up and running. Here is its export:

apiVersion: v1items:
- apiVersion: extensions/v1beta1
  kind: Deployment
  metadata:
    annotations:      deployment.kubernetes.io/revision: "2"      kubectl.kubernetes.io/last-applied-configuration: |        {"apiVersion":"extensions/v1beta1","kind":"Deployment","metadata":{"annotations":{"deployment.kubernetes.io/revision":"1"},"creationTimestamp":"2019-04-14T15:32:12Z","generation":1,"name":"frontend","namespace":"default","resourceVersion":"3138","selfLink":"/apis/extensions/v1beta1/namespaces/default/deployments/frontend","uid":"796046e1-5eca-11e9-a16c-0242ac110033"},"spec":{"minReadySeconds":20,"progressDeadlineSeconds":600,"replicas":4,"revisionHistoryLimit":10,"selector":{"matchLabels":{"name":"webapp"}},"strategy":{"rollingUpdate":{"maxSurge":"25%","maxUnavailable":"25%"},"type":"RollingUpdate"},"template":{"metadata":{"creationTimestamp":null,"labels":{"name":"webapp"}},"spec":{"containers":[{"image":"kodekloud/webapp-color:v2","imagePullPolicy":"IfNotPresent","name":"simple-webapp","ports":[{"containerPort":8080,"protocol":"TCP"}],"resources":{},"terminationMessagePath":"/dev/termination-log","terminationMessagePolicy":"File"}],"dnsPolicy":"ClusterFirst","restartPolicy":"Always","schedulerName":"default-scheduler","securityContext":{},"terminationGracePeriodSeconds":30}}},"status":{"availableReplicas":4,"conditions":[{"lastTransitionTime":"2019-04-14T15:33:00Z","lastUpdateTime":"2019-04-14T15:33:00Z","message":"Deployment has minimum availability.","reason":"MinimumReplicasAvailable","status":"True","type":"Available"},{"lastTransitionTime":"2019-04-14T15:32:12Z","lastUpdateTime":"2019-04-14T15:33:00Z","message":"ReplicaSet \"frontend-7965b86db7\" has successfully progressed.","reason":"NewReplicaSetAvailable","status":"True","type":"Progressing"}],"observedGeneration":1,"readyReplicas":4,"replicas":4,"updatedReplicas":4}}    creationTimestamp: 2019-04-14T15:32:12Z    generation: 2    labels:      name: webapp    name: frontend    namespace: default    resourceVersion: "3653"    selfLink: /apis/extensions/v1beta1/namespaces/default/deployments/frontend
    uid: 796046e1-5eca-11e9-a16c-0242ac110033  spec:
    minReadySeconds: 20
    progressDeadlineSeconds: 600
    replicas: 4
    revisionHistoryLimit: 10
    selector:
      matchLabels:
        name: webapp
    strategy:
      rollingUpdate:
        maxSurge: 25%
        maxUnavailable: 25%
      type: RollingUpdate
    template:
      metadata:
        creationTimestamp: null
        labels:
          name: webapp
      spec:
        containers:
        - image: kodekloud/webapp-color:v2
          imagePullPolicy: IfNotPresent
          name: simple-webapp
          ports:
          - containerPort: 8080
            protocol: TCP
          resources: {}
          terminationMessagePath: /dev/termination-log
          terminationMessagePolicy: File
        dnsPolicy: ClusterFirst
        restartPolicy: Always
        schedulerName: default-scheduler
        securityContext: {}
        terminationGracePeriodSeconds: 30
  status:
    availableReplicas: 4
    conditions:
    - lastTransitionTime: 2019-04-14T15:33:00Z
      lastUpdateTime: 2019-04-14T15:33:00Z
      message: Deployment has minimum availability.
      reason: MinimumReplicasAvailable
      status: "True"
      type: Available
    - lastTransitionTime: 2019-04-14T15:32:12Z
      lastUpdateTime: 2019-04-14T15:38:01Z
      message: ReplicaSet "frontend-65998dcfd8" has successfully progressed.
      reason: NewReplicaSetAvailable
      status: "True"
      type: Progressing
    observedGeneration: 2
    readyReplicas: 4
    replicas: 4
    updatedReplicas: 4
kind: List
metadata:
  resourceVersion: "

I am editing the metadata.strategy.type from RollingUpdate to Recreate.

However, running kubectl apply -f frontend.yml yields the following error:

Why is that?

Error from server (Conflict): error when applying patch:
{"metadata":{"annotations":{"deployment.kubernetes.io/revision":"1","kubectl.kubernetes.io/last-applied-configuration":"{\"apiVersion\":\"extensions/v1beta1\",\"kind\":\"Deployme
nt\",\"metadata\":{\"annotations\":{\"deployment.kubernetes.io/revision\":\"1\"},\"creationTimestamp\":\"2019-04-14T15:32:12Z\",\"generation\":1,\"name\":\"frontend\",\"namespace
\":\"default\",\"resourceVersion\":\"3138\",\"selfLink\":\"/apis/extensions/v1beta1/namespaces/default/deployments/frontend\",\"uid\":\"796046e1-5eca-11e9-a16c-0242ac110033\"},\"
spec\":{\"minReadySeconds\":20,\"progressDeadlineSeconds\":600,\"replicas\":4,\"revisionHistoryLimit\":10,\"selector\":{\"matchLabels\":{\"name\":\"webapp\"}},\"strategy\":{\"rol
lingUpdate\":{\"maxSurge\":\"25%\",\"maxUnavailable\":\"25%\"},\"type\":\"Recreate\"},\"template\":{\"metadata\":{\"creationTimestamp\":null,\"labels\":{\"name\":\"webapp\"}},\"s
pec\":{\"containers\":[{\"image\":\"kodekloud/webapp-color:v2\",\"imagePullPolicy\":\"IfNotPresent\",\"name\":\"simple-webapp\",\"ports\":[{\"containerPort\":8080,\"protocol\":\"
TCP\"}],\"resources\":{},\"terminationMessagePath\":\"/dev/termination-log\",\"terminationMessagePolicy\":\"File\"}],\"dnsPolicy\":\"ClusterFirst\",\"restartPolicy\":\"Always\",\
"schedulerName\":\"default-scheduler\",\"securityContext\":{},\"terminationGracePeriodSeconds\":30}}},\"status\":{\"availableReplicas\":4,\"conditions\":[{\"lastTransitionTime\":
\"2019-04-14T15:33:00Z\",\"lastUpdateTime\":\"2019-04-14T15:33:00Z\",\"message\":\"Deployment has minimum availability.\",\"reason\":\"MinimumReplicasAvailable\",\"status\":\"Tru
e\",\"type\":\"Available\"},{\"lastTransitionTime\":\"2019-04-14T15:32:12Z\",\"lastUpdateTime\":\"2019-04-14T15:33:00Z\",\"message\":\"ReplicaSet \\\"frontend-7965b86db7\\\" has
successfully progressed.\",\"reason\":\"NewReplicaSetAvailable\",\"status\":\"True\",\"type\":\"Progressing\"}],\"observedGeneration\":1,\"readyReplicas\":4,\"replicas\":4,\"upda
tedReplicas\":4}}\n"},"generation":1,"resourceVersion":"3138"},"spec":{"strategy":{"$retainKeys":["rollingUpdate","type"],"type":"Recreate"}},"status":{"$setElementOrder/conditio
ns":[{"type":"Available"},{"type":"Progressing"}],"conditions":[{"lastUpdateTime":"2019-04-14T15:33:00Z","message":"ReplicaSet \"frontend-7965b86db7\" has successfully progressed
.","type":"Progressing"}],"observedGeneration":1}}
to:
Resource: "extensions/v1beta1, Resource=deployments", GroupVersionKind: "extensions/v1beta1, Kind=Deployment"
Name: "frontend", Namespace: "default"
Object: &{map["kind":"Deployment" "apiVersion":"extensions/v1beta1" "metadata":map["annotations":map["kubectl.kubernetes.io/last-applied-configuration":"{\"apiVersion\":\"extensi
ons/v1beta1\",\"kind\":\"Deployment\",\"metadata\":{\"annotations\":{\"deployment.kubernetes.io/revision\":\"1\"},\"creationTimestamp\":\"2019-04-14T15:32:12Z\",\"generation\":1,
\"name\":\"frontend\",\"namespace\":\"default\",\"resourceVersion\":\"3138\",\"selfLink\":\"/apis/extensions/v1beta1/namespaces/default/deployments/frontend\",\"uid\":\"796046e1-
5eca-11e9-a16c-0242ac110033\"},\"spec\":{\"minReadySeconds\":20,\"progressDeadlineSeconds\":600,\"replicas\":4,\"revisionHistoryLimit\":10,\"selector\":{\"matchLabels\":{\"name\"
:\"webapp\"}},\"strategy\":{\"rollingUpdate\":{\"maxSurge\":\"25%\",\"maxUnavailable\":\"25%\"},\"type\":\"RollingUpdate\"},\"template\":{\"metadata\":{\"creationTimestamp\":null
,\"labels\":{\"name\":\"webapp\"}},\"spec\":{\"containers\":[{\"image\":\"kodekloud/webapp-color:v2\",\"imagePullPolicy\":\"IfNotPresent\",\"name\":\"simple-webapp\",\"ports\":[{
\"containerPort\":8080,\"protocol\":\"TCP\"}],\"resources\":{},\"terminationMessagePath\":\"/dev/termination-log\",\"terminationMessagePolicy\":\"File\"}],\"dnsPolicy\":\"Cluster
First\",\"restartPolicy\":\"Always\",\"schedulerName\":\"default-scheduler\",\"securityContext\":{},\"terminationGracePeriodSeconds\":30}}},\"status\":{\"availableReplicas\":4,\"
conditions\":[{\"lastTransitionTime\":\"2019-04-14T15:33:00Z\",\"lastUpdateTime\":\"2019-04-14T15:33:00Z\",\"message\":\"Deployment has minimum availability.\",\"reason\":\"Minim
umReplicasAvailable\",\"status\":\"True\",\"type\":\"Available\"},{\"lastTransitionTime\":\"2019-04-14T15:32:12Z\",\"lastUpdateTime\":\"2019-04-14T15:33:00Z\",\"message\":\"Repli
caSet \\\"frontend-7965b86db7\\\" has successfully progressed.\",\"reason\":\"NewReplicaSetAvailable\",\"status\":\"True\",\"type\":\"Progressing\"}],\"observedGeneration\":1,\"readyReplicas\":4,\"replicas\":4,\"updatedReplicas\":4}}\n" "deployment.kubernetes.io/revision":"2"] "name":"frontend" "namespace":"default" "selfLink":"/apis/extensions/v1beta1/namespaces/default/deployments/frontend" "resourceVersion":"3653" "labels":map["name":"webapp"] "uid":"796046e1-5eca-11e9-a16c-0242ac110033" "generation":'\x02' "creationTimestamp":"2019-04-14T15:32:12Z"] "spec":map["selector":map["matchLabels":map["name":"webapp"]] "template":map["metadata":map["creationTimestamp":<nil> "labels":map["name":"webapp"]] "spec":map["dnsPolicy":"ClusterFirst" "securityContext":map[] "schedulerName":"default-scheduler" "containers":[map["terminationMessagePolicy":"File" "imagePullPolicy":"IfNotPresent" "name":"simple-webapp" "image":"kodekloud/webapp-color:v2" "ports":[map["containerPort":'\u1f90' "protocol":"TCP"]] "resources":map[] "terminationMessagePath":"/dev/termination-log"]] "restartPolicy":"Always" "terminationGracePeriodSeconds":'\x1e']] "strategy":map["type":"RollingUpdate" "rollingUpdate":map["maxSurge":"25%" "maxUnavailable":"25%"]] "minReadySeconds":'\x14' "revisionHistoryLimit":'\n' "progressDeadlineSeconds":'\u0258' "replicas":'\x04'] "status":map["observedGeneration":'\x02' "replicas":'\x04' "updatedReplicas":'\x04' "readyReplicas":'\x04' "availableReplicas":'\x04' "conditions":[map["type":"Available" "status":"True" "lastUpdateTime":"2019-04-14T15:33:00Z" "lastTransitionTime":"2019-04-14T15:33:00Z" "reason":"MinimumReplicasAvailable" "message":"Deployment has minimum availability."] map["lastUpdateTime":"2019-04-14T15:38:01Z" "lastTransitionTime":"2019-04-14T15:32:12Z" "reason":"NewReplicaSetAvailable" "message":"ReplicaSet \"frontend-65998dcfd8\" has successfully progressed." "type":"Progressing" "status":"True"]]]]}
for: "frontend.yml": Operation cannot be fulfilled on deployments.extensions "frontend": the object has been modified; please apply your changes to the latest version and try again