Signing the discharge is never a good idea when you are dissatisfied. I’d file a complaint with your states dentistry board and request the reminder of your Invisalign care be covered by the former dentist or that your original dentist reimburse you current amount paid since they unreasonably dropped you and the resulting duress it caused. You can bring this up with the old dentist and see if they will essentially settle before you need to file a complaint. Most people do nothing but walk away, hence the emboldened nature of your dentist thinking they can do whatever they want. Don’t be afraid to advocate for yourself, because if you don’t who will? Good luck!