Move guest posts to user?

[angeljs]

After transferring my database via Impex, I stupidly gave 2 members the same username, but with different email addresses. The daft result is that now my main member's posts are all showing up as guest posts, but I can't merge them as they have the same username.

Is there a way to transfer/move all of the guest posts to the user of the same name?

I've tried changing the secondary username and merging them that way, but because the first user is classed as a guest, it won't let me merge them together.