You have to check the fdb processes running inside the storage Pods. It’s expected that during the upgrade (after all processes are restart to run the new version) the FDB cluster will have log/stateless Pods in 6.2 and 6.3, I described that here: How to upgrade apple operator 0.48 to 1.4.1 without deleting the old crd? - #27 by johscheuer.
What do you mean by the “connection string is old”? Have you done any manual operation on this cluster?
Like stated here: How to upgrade apple operator 0.48 to 1.4.1 without deleting the old crd? - #29 by johscheuer the first step is to use kubectl exec to exec into the Pods still running with the 6.2 image and see if they are still running the old fdbserver binary (that would be /usr/bin/fdbserver) or if they are using the new binary from /var/.. if they still use the old fdbserver binary you have to restart this process by using kill e.g. pkill -f fdbserver.