When things are messy enough that I want to rebase a branch, I generally create a new branch and new PR, and close the old PR with a comment linking to the new PR. I also put a link to the old PR in the description of the new one, so people can easily go back and look at the old discussion.
In other words, rebasing is a big enough change that I feel it's appropriate to restart the PR.
If you are linking the requests with a comment on each end, I'm not sure I see the benefit to just pushing the rebase into an open request. Is there a real difference?
In other words, rebasing is a big enough change that I feel it's appropriate to restart the PR.