If you don't already have an A/B or multi-variate framework in place, first push the no-effect change to the old code to understand that now it needs to be conditional on XX.
IMO, adding a new payment provider is a logical evolution. If your intent is to turn the old one off, I still believe it's worth the complexity to run them both until you're sure.
Besides, you should have your calls to your payment processor both abstracted and logged, so switching processors shouldn't change the way your application files payments. And you can review the logs to make sure things are still working, too. Which you should do anyway, code changes or not.