As long as you have a CC on file with PP that is not expired, you can change your payment method in your preferences:
Click on Profile > Financial Information > Pay List >Set available funding source
You can also change each transaction's funding source seperately, next time you go to pay for something, there is a link near the bottom of your PP page that says 'change funding source' and you can change it there.
I think it's only paying how you 'told' it to pay, if you know what I mean. But you can change it to any funding source you have on file with them.