Cannot change destination branch

I setup a simple pull request:

[forked repo] somebranchname -> [original repo] somebranchname (new)

But, when I create a new branch in a forked repo, then create a pull request for my new branch to a new branch with the same name on the source repo, I can't edit and change the target branch, it will get stuck on the "somebranchname (new)" branch. The only way I can change the pull request to another (existing) branch in the original repo is be creating a completely new pull request.