You can change your method of payment after the payment period of your currently registered method of payment ends.
To change your pixiv Premium payment period, if you currently have a monthly subscription, you can only re-register with pixiv Premium and change your method of payment after your premium member subscription period ends. Please be advised that you cannot change your premium account during a subscription period.
If you have a ticket subscription, you can change your method of payment within 1 to 15 days following the month that your contract expires.