Are your past customers out of sight and out of mind?
Doesn’t really matter why they left, those customers are still assets for your business. They can come back, refer others, influence others’ decisions. Your internal champion could take another job (or start another business) and want you back.
They’re no longer paying you, but that doesn’t mean you shouldn’t (to a degree) act as if they never left. Stay in touch, and continue adding value. Give them access to your content, check in on their progress & success from time to time, congratulate them on milestones.
Systematize these efforts and processes so that you’re doing it for everyone but appropriately scaling your required time and commitment to it.
No matter why they left, make it exceedingly clear that you still care about their business and their success. This costs you next to nothing, and the benefits can be huge.