do Upgrades only if there is a need to do. As you’ve seen there are a lot of releases so installing every release is work and may introduce a problem. (6.3 on CCR is worth the upgrade).
Normaly your config is there after upgrade (unless there is a problem).
Do backup with export compact. This gives a good readable config which you
can apply in a terminal with copy/paste. If routers/releases are different there might be
syntax/numbering changes you have to take care of. If you dont use “compact” take care of
MAC-Addresses. You dont want to change MAC Adresses.