Thanks for writing this up, I'm just at the tail end of having re-architected the CloudFlare billing system, also a subscription system written in Java with a MySQL back-end, but fronted by a Go API that insulates the rest of the business from the internals of the billing system.

The blog post covers a lot of the high level stuff really well, but I'm interested to learn whether you experienced any issues along the way, what they were, how you dealt with them.

In CloudFlare's case our migration was made more complex by also adding PayPal, and changing our processing gateway. Both of which created risk that we've had to work hard to understand and mitigate, i.e. how different gateways may, with the same card, return different results, etc.