I had to change multiple account names through WHM. After changing account names I found out that MySQL databases with InnoDB screwed up, throwing following error:

Can't open file: 'example_config.ibd' (errno: 1)

I was googling alot on this matter but none of the fixes I found worked for me. So, to save the day here's what you got to do:

1) Change back to old username (using WHM's modify account)
2) Make dump of your database, add this in beginning of the dump: SET FOREIGN_KEY_CHECKS=0;
3) DROP the whole database (Dropping only tables might work, I havent tried it)
4) Change username to the new username (using WHM's modify account)
5) Restore database from the dump.
6) All set, your database should work just fine with no errors.

If this doesnt help you, take a look at this post in MySQL forums: http://forums.mysql.com/read.php?22,34050,52933#msg-52933