So what do you do when you want to make a change that isn't a logical evolution of your existing codebase? Say, for example, you have an ecommerce site, and you're switching payment processors to use one with an entirely different API. Surely you can't just push a change like this straight to your production server.
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.
Yep, after all the tests run. I'm sure you have good test coverage on your payment stuff, right?
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.
