The question seems to be about supporting two versions at the same time. That is kind of Canary deployment, which make production traffic to gradually shifting from app-v1 to app-v2.

This could be achieved with:

• Allow deployments to have HPA with custom metric that based on number of connections. That is, when it reaches certain number of connections scale up/down.
• Allow two deployments at the same time, app-v1 and app-v2.
• Allow new traffic to route on new deployment via some Ingress annotation, but still keeping access to the old version, so no existing connection be dropped.
• Now, all the new requests will be routed to the new version. The HPA eventually, scale down pods from old version. (You can even allow deployment to have zero replicas).

Addition to your question above blue-green deployments. The blue-green deployment is about having two identical environments, where one environment active at a time, let's say blue is active on production now. Once you have a new version ready for deployment, say green, is deployed and tested separately. Finally, you switched the traffic to the green environment, when you are happy with the test result on green environment. So green become active while blue become idle or terminated later sometime. (Referred from martin fowler article).

In Kubernetes, this can be achieved with having two identical deployments. Here is a good reference.

Basically, you can have two identical deployments, assume you have current deployment my-deployment-blue is on production. Once you are ready with the new version, you can deploy it as a completely new deployment, lets say my-deployment-green, and use a separate test service to test the green environment. Finally, switch the traffic to the my-deployment-green when all test are passed.