I agree this is something that still needs more thought as it can be complex. Overall I would say to try to avoid migrating inflight processes unless you really need to. You can redirect new requests to new versions ( completely new runtime/service instance ) and let old instances complete.
Another possibility with the current API is to start a new process instance using the REST API and passing a X-KOGITO-StartFromNode header, that way you can copy the state of the current process and send as the state of the new instance, restarting the process from a specific point/node.
This area is something that will be looked more into details of upcoming releases.