Medicare Supplemental Insurance - Switching Companies Possible?

My mother called Physicians Mutual and the salesman told here that once she chose them, that she cannot change companies ever. Is that true? He told her verbatim that "Once you choose us, you cannot ever change to another company." Is that true? She has been on the phone with various companies for weeks. She is looking for the cheapest premium as it seems that all plans are the same by law. She is leaning toward Plan C. Any ideas which company is the cheapest? And can she switch companies once she has chosen one? She lives in Tennessee.