[sokoloff]

Make the code change conditional and push it out off, cookie (or query string or URL) force some users into the new path...

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.