Officially it cannot be done. Unofficially, you might just have to offer a large enough bribe to the right people.... Perhaps if you bribe me enough I can tell you who those people are...
http://forum.grasscity.com/general-forum-feedback/1007341-how-change-username-2.html#post14165721 Sometimes the admin will make small changes for people but it's not a priority so it may take a while. The fact that it's what you use for everything is a good reason to either ask for a change or get this account deleted and make a new one... If anyone who you don't want knowing about your habits (family, employer, etc) searches for Mattbo4 on Google, they're going to find your GrassCity account.
Very true about the family and employer thing. Not sure what to do here, and I dont want to make a new account due to the fact I just purchased a bronze membership and my blog entries.
They could probably work out a way to transfer over your bronze membership. Anyhow, you could always send a PM to the admin titled "User name change request" or something like that, and ask her to change it to... I don't know, Mattro7 or something else similar to your current name but different enough to not cause you a Google problem. The main thing is just that you still need to be recognizable, so changing from Mattbo4 to SuzyQ15 isn't an option. Never hurts to try. At the very least, don't send out any job applications with a Mattbo4 email address.