No. However, many parties to a settlement recommend annuities as a way to provide the injured party with security of future payments. Medicare does allow for MSAs to be annuitized and will review and approve the seed amount (initial funding provided to the injured party) and annual payment amounts.
If the MSA submitted to Medicare is approved as a lump sum, then it cannot be changed to an annuity unless it is re-submitted for approval. On the other hand, if the MSA is approved as an annuity, the parties to a settlement can decide to change to a lump sum without notifying Medicare.