The fromMigrate flag has been added to the DurableReplOperation class (IDL-defined) in 6.1 and this will be backported to 6.0 also (see SERVER-65858).

The procedure for downgrading the replica sets requires 1) downgrading the FVC and 2) downgrading each binary starting from secondary nodes. This implies that it exists a time window in which the primary node uses a new binary where the secondaries use an old one.

Downgrading from 6.0 to 5.3 could cause a crash of secondary nodes (using 5.3 binary) caused by an unrecognized field in the oplog entries, i.e., the fromMigrate flag populated by the primary node (using 6.0 binary).

The goal of this ticket is to avoid to set the fromMigrate flag of DurableReplOperation objects when the current FCV is lower than 6.0.