OK, reading those it seems that s2M should do what i want, and not what it does:
The new terms will take effect at the end of the current billing cycle. In other words, if a Customer signs up for a monthly Subscription on Jan 1st at $9.99/mo… then they upgrade on Jan 15th to $99/year; they will be billed $99 on Feb 1st and be re-billed Feb 1st of each year.
This means that effectively, it should be updated on the next scheduled payment, right? (as i am using the Paypal Standard integration).