If you stop paying private insurance premiums the contract can't be terminated (you still accrue debt) though at some point they will switch you into the Notlagentarif which offers much reduced benefits. It will cover all acute pain treatment and chronic diseases but that's pretty much it.
As soon as you are older than 55 and have been privately insured for at least the last five years, the only possibilities to switch from private to public insurance are the following two:
- You move into another European country with compulsory insurance for more than 12 months. After moving back to Germany you have a three-month window to enroll in German public insurance.
- Your spouse is publicly insured and you fulfill the criteria for family insurance (including earning less than 415 € a month, or 450 € in case of a mini-job).
There are also some options at your private insurer:
If you earn less than 50 850 € and have been insured for more than 10 years you can switch to the Standardtarif which has similar benefits as public insurance. It has a price cap 618,68 € a month although most insured need to pay much less.
There is also the possibility to switch to the similar Basistarif which has a price cap of currently 665,29 € per month.
You should also discuss other options with your insurer.