Quest Software Support recommends to avoid changing any of the Service Account passwords midst migration. This applies to both Migration Manager for Active Directory and Exchange. Doing so will result in a disruption of the migration process and potentially unexpected results or behavior may be encountered.
It is recommended to set all Service Account passwords to never expire within Active Directory.
While gMSA will find and change services passwords, it does not have the capability of changing passwords within the Migration Manager toolset and concurrently the ADLDS databases and SQL database involved in the migration scenario.