Not many people are willing to change their gamertags. Maybe you should just sack that completely and make it optional. Not many people like being forced to change their gamertags, for things like a clan. Even some of the pro gamers back in the day never changed their gamertags to resemble what team they played for.
We have decided to put this in. We are only interested in loyal and dedicated members and this shows us that.
The most loyal members are the ones that get on everyday, and play with everyone. I am in my own clan and we have has some of the most loyal members and they never changed their names. I have been in my clan for just about a year now and I have seen many people change their names and eventually leave. Make it an option for those that want to do it, and some people might be more dedicated and loyal because of it.
It just seems cruel to force someone to do something that they don't want to do. I think it is good enough that they are trying to join, and are putting themselves out there.