http://protonmailrmez3lotccipshtkleegetolb73fuirgj7r4o4vfu7ozyd.onion/support/changing-currency-for-paid-plans
Depending on how long the plan has been used, you will be only charged the difference for the extension. For example, if you pay monthly on 10 August, your paid plan will last until 10 September. If you then decide to change the currency on 20 August, your plan will be extended until 20 September, but you will only be charged for the 10-day difference, not the full amount for one month.