It's 2015... your stakeholders have decided that PayPal is out and Stripe is in. Fortunately, you're a planner. You have all of your PayPal interactions isolated in an adapter gem and your interface is clearly defined, easy to re-implement and enforce. Rewriting your adapter isn't small, but it is surmountable and can be estimated accurately so it won't lead to surprises. You win.
Larger Rails apps are moving towards Service Oriented Architecture with adapters (typically custom gems) abstracting 3rd party services. This talk will cover defining, enforcing and test-driving the interface for your 3rd party service to provide easier flexibility in the future.