[suggestion] Add early renewal

[nehemiagurl]

Motivation

Last year I pledged to support some organization, and put in a very large down payment that covered my pledge years in advance.
Recently I wanted to upgrade my pledge, but I noticed that upgrading doesn't ask me for payment details but only changes the date of the next renewal. Trying to cancel and renew the subscription lead me to the same place.
Now, I don't think this is necessarily bad behaviour. Committing to a bigger payment in the future doesn't necessarily mean you agree to a bigger payment now. Cancellation and renewal taking into account the last pledge also makes sense in my opinion.
Therefore, I think the best solution will be to add a "renew early" sort of button to the "modify" page will be a great solution. It will also be of general use - sometimes people just want to renew early.

What should it behave like?

There's many ways I can see this sort of button going, but I think that to simplify things and to make for an easily understandable behaviour, it should act like how early renewal on most commercial subscriptions work, i.e. you pay now the amount it would cost you were you to buy now for the first time, minus whatever is left in your "balance".

Demonstrative example

Say I donate $50 a year to Liberapay, and that I've set a 4 year down payment the first time I donated. After 3 years I want to renew, once again with a 4 year down payment. Since the "balance" I have left is $50, I will now pay $150 and the next renewal date will be in 4 years.