On what tokens? The ones of today's model, or tomorrow's?
The API makes it simple, yes. They make you pay per token per model. A new model has different prices. Not being able to do that is what makes it not so very simple.
Are you serious? If you used 10$ of your subscription until 10th of month and then prices changed, well, the rest of your subscription uses new prices, that's basic math.
Just say you get $100 of credit and different models cost different rates.