Hi,
That's correct its due to SidHistory. You wouldn't want two different users with the same name having the same document history....
You can try using STSADM –o migrateuser Todd has a pretty good walkthough and diagram of how it works. However, you ar3 supposed to plan for it and I am not sure now that the oldusername and the newusername are the same is it will work properly...
But it wont hurt...
Reference:
http://www.toddklindt.com/blog/Lists/Posts/Post.aspx?ID=75
-Ivan