I did need to do one manual step.
I told the upgrade to leave my extant /etc/mongod.cnf and discovered on restart that the key storage.journal.enabled seems to have become invalid since 6.x
It is always a good idea to follow the instructions on MongoDB site when upgrading.
(if you have a replica set or a sharded cluster, there is a dedicated page for each).
Carefully read all the steps, even if you previously upgraded from an older version as there might be differences. Failing to follow all the steps or ignoring one of them can lead to bad things in production. Don’t ask me how I know