Edited pledge - Entitled rewards

I often get confused when my patrons edit their pledge to a higher one because, I have to check if they have already received the rewards for the previous pledge that month ( I’m charging patrons monthly up front ). If they have received their rewards for the previous pledge, my position when they ask for the higher tier rewards the same month is a negative, as they have only been charged for the difference from the previous pledge. Do you think I am acting right?
In any case, they appear in both tier benefits for that month, having only paid for the price of the highest one. Am I the only one confused about this?