I have a Debian
with 12 containers running on it (all using Debians with different versions).
I now want to upgrade the server to Debian
lxc/lxd 5.02 natively)
Will this most likely go smoothly or do I need to take care because of incompatibility?
LXD doesn’t support downgrading, so this will go very badly.
You’re going to have to setup a new empty LXD and then move your data from the newer LXD, either by using lxc copy/move over the network or by using export/import for each instance.
(Also, LXD is no longer part of Linux Containers, support for LXD is on the Canonical forum these days).