If you edit commits in the branch, then yes, it appears as declined in the UI, which looks hostile when they are notified. I write a friendly message every time I do this, but it sucks to have to give that explanation and the UI metadata is still wrong. I asked for this on bitbucket, though it's currently on hold [1].
If you don't edit the existing commits, then it will show up in the github/bitbucket UI as merged when you eventually merge, regardless of what games you play with branches.
One way I've worked around this is to merge the original pull request, then immediately force-push the fixed version. Still doesn't fix all of the UI metadata, but at least the pull request gets marked as merged.
Only works when you have push access to the repository that is the source of the PR. Normally maintainers don't have access to the repositories for all contributors.
Er, no? You merge the unfixed version via the web UI (to get the PR marked as merged), then immediately force-push the fixed version to your own repo, not their's.
Oh, I thought you meant updating the pull request by force-pushing on their branch. Your solution is racy, thus only acceptable for projects small enough that the likelihood of someone pulling during that window is sufficiently low.
I meant that it would be useful to add a function for that workflow in the github UI.