Allow changing of billing method with Nitro subcription

You currently cannot swap to a new credit card if you have an active Nitro subscription. This becomes problematic when someone wants to change their billing method and/or credit card. For example, I closed a credit card and want to bill my Nitro to a new one. Discord doesn't allow me to swap while my Nitro is still active. Instead, I get an email daily saying that my old card is invalid and that the payment didn't go through, and to update the info, which isn't possible.