42 CFR §409.66

1. (a)Except as provided in paragraph (c) of this section, a beneficiary (or anyone authorized to execute a request for payment, if the beneficiary is incapacitated) may revoke an election not to use lifetime reserve days during hospitalization or within 90 days after discharge.
2. (b)The revocation must be submitted to the hospital or CAH in writing and identify the stay or stays to which it applies.
3. (c)Exceptions. A revocation of an election not to use lifetime reserve days may not be filed—
   1. (1)After the beneficiary dies; or
   2. (2)After the hospital or CAH has filed a claim under the supplementary medical insurance program (Medicare Part B), for medical and other health services furnished to the beneficiary on the days in question.