> But, my fear is it'll subtly break something and I just don't have enough hours left in my life to accept yet unknown risk that it'll cost me even more hours,
Yeah, it's not like 99% of the world has already switched from master to main already (without any major problems) ...
You'll find CI/CD automation probably needs to be updated. (Triggering different actions when merges to the default branch happen, or perhaps just deployments.)
These are the kinda local things that the parent was probably referring to.