Changing the PVC size in spec resulting in new pods being created and the old pods still remain

So, just need to wait. Is that correct? Which version of operator support this?

I believe all versions of the operator should support this.

On a separate question, what if someone removed the pods manually BEFORE the data movement finished and resulting in UNHEALTHY: No replicas remain of some data. Is running restore the only way to recover?

Since version 1.2.0 those Pods are recreated until they are fully excluded (data movement finished): Release v1.2.0 · FoundationDB/fdb-kubernetes-operator · GitHub

What if say, I changed the size, and the new storage pods got created. And before it can finish migration, I deleted all the old storage pods and now the DB is telling me it is in a “Unhealthy state, some data don’t have any replica”. Is there a way to clean the db of all the data and put it back into a healthy state so that I can do a restore?

Like above the operator (1.2.0+) should recreate those Pods again and the cluster should be able to finish the data movement. You you to remove the whole key space, see this post for the command: Can't clear database (delete all data)