[phildenhoff]

If you want to “checkout” some previous commit, jj has your back in three ways

- first, that commit that’s been merged to main is marked as immutable and, unless you add a flag to say “I know this is immutable and I want to mutate it anyway”, you can’t mutate it

- second, as part of your regular workflow, you haven’t actually checked out that historical commit. You created a new, empty commit when you “checked it out” using “jj new old_commit”

- third, you can use jj undo. Or, you can use “jj obs log” to see how a change has evolved over time (read: undo your mass find+replace by reverting to a previous state)