If someone changes their name, they should make certain they know what they've changed it to. In the event they forget it, there's lots of good feedback in this thread (like noting reg id. as mentioned above) to help find the name. Failing that, creating a new account and asking an admin in-game or using the forum help request section is an option.
We've had a "damned if we do, damned if we don't" reaction to some changes in the past and wouldn't want to upset the frequent name changers amongst the crowd, many of which have no problem remembering their new nicknames.
For those reasons, we won't be adding a limit.
Thanks.