I'm going to assume that you can't just add new subscribers b/c of their credit card information. There can be some PII + PCI complications.
I think I have a work-around that you could use.... It's a bit janky if you've got customers who have different start dates. You may have to manually manage that.
You can build a discount making membership levels free. You could migrate your customers by having them sign up and giving them free coupons. You can provide a discount that's 100% off. Just be careful of how you set it up so you don't give them free membership forever (or DO, in the case of the doctor's office).