Pull requests to a closed branch cannot be updated to point to an open branch (BB-9464)

• Created a pull request (B) to merge into an existing branch (A).
• (A) also had a pull request pending to go into our dev branch.
• (A) was approved and merged into develop and has been closed.
• So now (B) WOULD be trying to merge into a closed branch - because the branch is closed bitbucket shows an error that the diff cannot be created.

I tried to edit (B) to be a pull request into develop instead of into the closed branch (A) - but I see the following error on submitting my changes to the pull request(B):
The following error(s) occurred saving this pull request. Can't create branch feature/view-payments since it already exists in Carleton (Android).

I have no way to delete or update B, and since A is closed there is no diff available to properly review B.