Apparently when you subscribe to a lower tier plan such as the one month one, the amount you paid (minus what you already “used”) will be deducted from the upgrade price if you decide to commit for a longer amount of time.

This is great for people like me who want to try out all the features (including custom domains, Proton Bridge and SimpleLogin) without having to commit for one or two years and without having to pay the 65% higher price of the monthly plan.
I had already accepted the fact that I’d need to pay the monthly fee but this is a much appreciated gesture!