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.
This talk will cover a collection of best practices for defining, enforcing and test-driving the adapter for your 3rd party service to provide easier flexibility in the future.