Changing the target branch of a pull requests that is under review should cause Skara to send an RFR email to the mailing list. For repos that have "branchname" configured in the bot, that would mean the subject will change to include the branch name as a prefix. This is a very important indication to reviewers about the release that the PR will go into, which could otherwise be missed.

Here is a recent example, where the branch changed, but I missed that he had done it until I did a query of open PRs targeted to jfx20:

https://github.com/openjdk/jfx/pull/993

In this case I was expecting it, but if I hadn't been, it might have been a problem.