I tried to get information from FMCA by submitting my question about plan expiration, via their web form. Alas, after waiting for more than a week, no one responded.
After a bit of navel gazing, I realized that my actual contract for the cellular service was with Verizon, not FMCA.
I spoke with Verizon support this evening and asked what happens when my contract expires next month. I was told that my coverage will continue at the same price, after my contract expires.
That's good news to me. Verizon data, at a reasonable price, with the freedom to terminate the service at any time. No contractual or membership requirements.