Look, I read some forums searching for the answer to your question. Most posts I found people confirming that you can renam rename your character to a name you used before, but you need to wait 6 months until the old name is not related to your character.
Example: If your former name was X and you change to Y. The name X will be related to your character on official site and when you look for Y, the name X will appear as a former name. So after 6 months, the name X won't be related anymore and you can go back.
Note that this is only answers I've found in forums.