Hi there! I apologize for the confusion; due to the version of VPS we were offering at the time you ordered yours, it does not have the option of switching immediately to an alternative billing plan.
No worries, though! Cost-wise, if you order a new VPS in your web panel (select the yearly billing option) and select to have your sites and users moved to it, it will be the same as changing the billing plan. Your account is credited a pro-rated amount for the rest of your current billing cycle, and the new yearly plan will start the day you make the move.
We offer a variety of payment methods including Checking Account, Check, and Money Order, with credit cards being required at signup and for domain registrations.
I sent you an email with this information as well, please feel free to reply there if I can still help!