Normally, if you want to remove the default Gmail accounts from Settings -> Accounts & sync -> Remove the default account, you will prompted with error message “This account is required by some applications. You can only remove it by resetting the phone to factory defaults (which deletes all you personal data). You do that in the Settings application, under Privacy.”. This will cause you files and setting will deleted include the paid one.
Here I want to share on how to change main Gmail account without doing a factory reset. Actually you can easily do this if your phone or tablet is rooted: