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:
The step to is:
1. Please do backup you files 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
(You can just name it back if had a problem or want to use the previous account instead of doing a restore)
5. Reboot you phone or tablet
6. After boot up, 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
Hope this will can help.
**I would definitely recommend doing a backup before, just in case you delete the wrong file accidentally.