42 CFR §407.50

1. (a)Deemed enrollment. When coverage under a buy-in agreement ends because the agreement terminates, or is modified to substitute a narrower buy-in group, or because the individual is no longer eligible for inclusion in the buy-in group, the individual—
   1. (1)Is considered to have enrolled during his or her initial enrollment period; and
   2. (2)Will be entitled to SMI on this basis and liable for SMI premiums beginning with the first month for which he or she is no longer covered under the buy-in agreement.
2. (b)Voluntary termination.
   1. (1)An individual may voluntarily terminate entitlement acquired under paragraph (a) of this section by filing, with SSA or CMS, a request for disenrollment.
   2. (2)Voluntary disenrollment is effective as follows:
      1. (i)If the individual files a request within 30 days after the date of CMS's notice that buy-in coverage has ended, the individual's entitlement ends on the last day of the last month for which the State paid the premium.
      2. (ii)If the individual files the request more than 30 days but not more than 6 months after buy-in coverage ends, entitlement ends on the last day of the month in which the request is filed.
      3. (iii)If the individual files the request later than the 6th month after buy-in coverage ends, entitlement ends at the end of the month after the month in which request is filed. 1