Smashwords is now saying that some accounts will be automatically moved to Draft2Digital. A May 1 email says:

On June 1st, we’ll begin automatically migrating some Smashwords author accounts to Draft2Digital. If you desire more control over the migration process, or want to have your account migrated sooner, please complete the brief questionnaire that was previously sent to the email address associated with your Smashwords account.
 
When Draft2Digital acquired Smashwords in March 2022, we announced that the integration of the two companies would be completed in three phases, with the final phase involving the migration of Smashwords author/publisher accounts over to Draft2Digital. Given the vast number of Smashwords accounts, the migration process itself must be divided into phases to ensure maximum effectiveness and efficiency.

The migration FAQ says that certain categories of authors, including affiliates and authors with books in multiple series, aren’t eligible to migrate “right now.” So it appears that migration may be compulsory, optional, or currently forbidden depending on each author’s details. My guess is that all accounts will eventually be migrated, but I haven’t found anything that says so outright.