Backwards incompatible changes to persistence always require at least two upgrade steps to encapsulate migration without causing downtime.
- Create a new REST service artifact "B" which creates the field and treats it as optional in read operations but required in write operations with an incremented minor version.
- Create a migration artifact (or include migration in the artifact from step 1) which populates the field. A/B deploy "B".
- Create a new REST service artifact "C" which treats the field as required with an incremented minor version.
- Wait until the migration is complete, then A/B deploy C.