For my Samsung Moment from Sprint I wanted to change the main Google account without factory defaulting the phone. I followed the instructions found at http://androidforums.com/ally-all-things-root/337386-remove-main-google-account-android-without-doing-factory-reset.html.
The core is to rename /data/system/account.db to /data/system/account.db.bak. You will ten need to enter the new account info when you attempt to access any of the google services – for example Market.