From the Model Mayhem FAQ:
If I upgrade my existing subscription, does my previous payment get pro-rated?
In short, yes!
Our billing system will automatically calculate the pro-rated amount you owe based on the amount you have already paid and how much time you have already used.
The system might initially show you the full price for the upgrade, however, when you go to process your payment the system will see your other payment already in the system and only bill you for the pro-rated amount.
It's easiest to use the monthly payments as an example:
A Premium subscription for one month is $6. A VIP membership for a month costs $10. Let’s say you have a monthly Premium subscription, and you decide to upgrade to VIP halfway through your month. You've already paid $6 of the $10, but since you've already gone through half of the subscription, you only have half of the time left to use VIP - which means instead of paying the remaining $4 (which would give you a full month of VIP), you pay $2 - or half, which covers the last half of the month you will be using VIP.