If you're within the time frame that you can edit a post you could edit your original/top post and change the thread prefix.
You would have to ask a moderator to do it. It's not that critical of an item and should not affect the activity if the post has a good title for the question.
For the record, you can use the 'Report' button under your post to send a message to the mods. That's usually done to notify us of a problematic post from someone else but it can also be used to ask us to modify your own post if you can't edit it.
Also note that you don't have to explicitly change the prefix to 'Resolved' when you issue is resolved. There's a dedicated button at the top of the thread on the right to do that.