It can be be done because i did it on an old site of mine a few years ago.
here are the instructions I used to do it.
"@SHaun,
Say you have 3 account types. Red, Blue and yellow.
Create a new user called reduser and make him a red account type, same for the blueuser and yellowuser.
Once you have created these accounts open up phpmyadmin (or however you can to see your database)
Look in the table ow_base_user
Near the end you will see the 3 accounts you created, reduser, blueuser and yellowuser.
In each line you see at the end the account type and a big long number.
Each number represents the account type which will be different for
red, blue and yellow.
All you need to do is to take a note of the number of the account you
want, copy the big number and paste it into the account type area of the
user you want to change account types for.
***BACKUP your site and database before you do any work on it.***"
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