You can upgrade the server on the billing site at http://billing.guildlaunch.net/ - the system will discount the server (so if you have the server because of an Ultimate subscription, and upgrade to 100 slots, it will cost as much as a 50-slot server to upgrade).
If you decide to switch to an annual plan, we'll handle the switch for the server on our end. :)
It depends on how the server was configured - a lot of them are configured to invoice for $0.00 on a monthly basis, since that's the default option. If you send an email to email@example.com from the address it's linked to, I can look it up and see if it's configured monthly or quarterly.
We use PayPal to process our payments, but you can make payments with a credit card that wouldn't involve creating a new PayPal account. :)