I find this idea very interesting, the inclusion of the ID of the users who are voted for.
It is true that it will be more work for those who vote, but it will guarantee the votes for those who change their username.
I can guarantee more people will mess up if they have to jump through hoops. Some already write names incorrectly, but it's still obvious who they mean. If they add the wrong userID, it gets worse.
Usernames are rare, and there's no doubt who the vote was for.