Variation of rebasing

Hi Martin,

maybe rebasing a branch of an upstream repo onto a PR of a downstream repo, might not be the best idea.
As I understand all of the commits of the upstream branch up to the common ancestor of the two branches will be replayed on top of the last commit of the feature branch, hence changing the history of an already published repo.
Please correct me if I'm completely off on this one.

Cheers,

Martin