How to change main Gmail account without doing a factory reset. Actually you can easily do this if your phone or tablet is rooted:
The step to is:
1. Please do backup you files/data or system using any suitable backup software.
2. Open your Root Explorer
3. /data/system/ and find a file with name accounts.db
4. Rename the accounts.db file to accounts.db.bak
5. Reboot you phone or tablet
6. After boot up, setup new account with go to Settings -> Accounts and sync -> Add account and now you can add new Gmail account. You also can sign in new Gmail account from android Market