If you haven't already cancelled your old subscription payment in your Paypal account you will need to do it ASAP, cancelling it will not affect your access to the forum.
Login to your Paypal account and click on the cog icon top right
Go to Automatic Payments
You will see a list of authorised payments, find the one for SMF and cancel it.
You will know your subscription is due when you logon to the forum and don't have all the usual options, at that points go to your user settings
(click on your username top right) you will then see a subscription tab, select the one you want and follow the instructions