If you use git reparent-branch, it will update the upstream ref for your branch and then try to rebase all branches, effectively running git rebase-update. This is very bad if you have stale branches as you have to either abort the rebase (especially slow on Windows) or fix merge conflicts. A better option would be to only rebase the current branch or not rebase at all.

P.S. I know you can mark stale branches using git cl but if you forget to do so you'll waste a lot of time when you run reparent-branch.