For upgrading I would suggest doing 6.2 → 6.3 → 7.1 and not directly 6.2 → 7.1 (I’m actually not sure if this is supported and well-tested). The following doc explains how an upgrade can be performed: Upgrading FoundationDB · apple/foundationdb Wiki · GitHub, make sure that your clients have both library version ready before doing the actual restart/upgrade of the FDB cluster. There are also some older forum post around upgrades in general: Upgrading FoundationDB. If your cluster is running on Kubernetes you might want to look at the operator docs to understand how the operator performs the upgrade: fdb-kubernetes-operator/technical_design.md at main · FoundationDB/fdb-kubernetes-operator · GitHub.
One of the limitations for version upgrades is (at least for major/minor upgrades) that those versions are protocol incompatible and you have to upgrade all fdbserver processes “at once” otherwise they are not able to communicate.