Can I go back to paying monthly after buying a year?

Once the year has actually started, yes, at the boundary like any plan change. In the window where the year is paid for but has not begun yet, the screen refuses and says why instead of letting you click — moving then would end the arrangement holding your prepaid year and you would lose it in one press. Undoing a paid year is not a button in either direction: the invoice for it is a numbered tax document, so reversing it means issuing a credit note against that document. Message support and it gets sorted properly.