AIPRM offers plan stacking. That makes upgrading specific quotas to fine-tune the exact amount possible. It can help users to minimize any waste or unnecessary expense.
So, for example, if someone needed double the quota of Private Prompts that the AIPRM Plus plan provides, they would not have to upgrade all the way to the AIPRM Pro plan, but could instead stack 2 AIPRM Plus plans, saving 50% of the cost of a Pro Plan for that one feature level they needed (though obviously not getting the extra features that only come with an AIPRM Pro Plan or above, such as custom tone/styles.)
Consequently, we do not follow the typical upgrade scenario where users move from a tiny plan to a small plan, and then to a large plan.
In cases where a user only needs a larger plan and not a smaller one he had before, users must cancel the smaller subscription, and can use it until expiry.
AIPRM does not provide partial or full refunds of any kind, even in this example of "upgrade scenario" with the intention to have only 1 active plan. This upgrade situation is also explained here.