There needs to be a way to top up payments, then in your example if the dev if the dev converted a free to a 10$ game with purchase to donation conversion the 1$ donator would then be able to top up with 9$ to reach the 10$;
or if a dev released a version 0.5 game for 6$ then added a tier of 7$ for version 0.6 the customer could just top up the 1$ instead of having to rebuy the game at full price again.