When a user re-opens a bug that has been marked resolved, he or she should be able to change the fix-for version (assuming appropriate permissions). This is important because the bug might be marked fix-for a version that has already been released. In this case, it's too late to fix it for that version, and the re-opened bug might be missed in filters of bugs for upcoming releases.

(PS This just happened to me – I didn't see the re-opened bug until too late.)