We launched a SaaS product on the 2016-03 API version. Since then[1] the API has basically changed completely around subscriptions (even the top-level objects aren't called the same anymore), to the point it would probably be easiest to just start a rewrite from scratch (which probably would take a week or two). But the fact that the old API continues to work to this day and that we have been able to rely on it and focus on other things has been amazing. New customers get to enjoy the new API while our business continues to chug along with zero modifications to the payment parts. That is impressive from both a technical and from a business point of view, and ensures we will keep using Stripe forever.

[1] Stripe provides a very detailed and customized API upgrade guide based on your current API usage https://stripe.com/docs/upgrades?since=2016-03-07#how-can-i-...