SERVER-74032 introduced the new transfersFirstCollectionChunkToRecipient optional field to the schema of MigrationCoordinatorDocument to generate the expected change stream events in case of donor step down.

Such a schema is currently protected by a strict constraint, so that the persistence of this new field must be FCV gated - and inflight migrations drained upon FCV downgrade.

Test regressions revealed that the implementation of such protections is nevertheless flawed: when the donor sets a commit/abort decision on the migration, the recovery document receives an update that includes the backwards incompatible field, opening the path to the following scenario:

• At commit decision time, the primary node of the donor (running on a 8.2 binary) persists an update  of the recovery document including the transfersFirstCollectionChunkToRecipient
• An election occurs
• The new primary (running on a 8.0 binary) runs the migration recovery routine - and crashes due to an unmanaged DBException due to the inability of parsing the recovery document