> f I have branch fix_something that is branched off master from a while ago, do I need to do anything to it?
In general no, branches are basically a file that points to a given commit so even if they branched from master the actual
commit sha is what matters. The only case if you have a branch that is tracking master as a remote, in which case you need
to do those steps for said branch. But this is quite uncommon.
> Or will I just be able to do `git merge main` once master gets renamed, and it'll just work with the name change making no difference?
You should be able to do that without any problem that I can foresee once you rename it in your local fork.
> PS If I think hard about it, and ponder the git model of blobs and commits, etc, I can see that no commits change with the renaming, and
> the names "master" and "main" are just aliases for particular commits.
So I think I've convinced myself that nothing else is needed. But that
> thought process is what I mean by "not obvious" ;-)
Exactly, that's the way I reason about this as well