Well you will have to manually upgrade then, since only users with unmodified scripts of Mys v1.2.2 can use the upgrader. Id advise you to look into the files functions.php, admin.php, account.php, register.php, login.php and find the differences between Mys v1.2.2 and v1.2.3's codes.
Well to do this, you will need to set up a new column whose value automatically increments by one if the user adopts a new pet or receives a baby adoptable from breeding center. You will then have to write codes that check users number of pets owned and decide if they can adopt certain special adoptables based on this value. Shouldnt be hard, but can be tedious.