Per staff request, I'm posting this as a standalone feature request:
Please allow moderators to change the outcome of a handled flag, such as when they accidentally/prematurely decline a flag they ultimately want to mark as helpful, instead (or vice versa).
This is a minor issue, but is one that continues to crop up on a regular basis. As I'm not a moderator anywhere, I can't provide any kind of stats on this, but it's something that moderators ought to be able to do, especially given they are supposed to be "exception handlers".