I don't know about MGC, honestly. I never use it. For True Achievements I think I just sent them a message and they switched it over for me. There might be a delay of a day or two before everything is back to normal, but nothing major.
Thanks. Might look into subscribing. I always see people with awards for playing in subscribers tournamnets, so that could be fun. Just another question though. Do you know if sites like MCG.net and TrueAchievements will identify the changed gamertag and simply carry on in my existing profile. (I did try setting up new accounts on the sites but i can't have more than one profile per email and i cannot change the e-mail address associated with my gamertag).
You actually have to be a subscriber to get your forum log-in name changed. I was a subscriber for about a year before I forgot to recharge my PayPal account and I accidentally let it lapse. I've been meaning to resubscribe, but I just havn't gotten around to it yet.