Before we can back-fill the new schema, we want to start writing to the new schema. This means setting
$wgMultiContentRevisionSchemaMigrationStage = SCHEMA_COMPAT_WRITE_BOTH | SCHEMA_COMPAT_READ_OLD;
For new, we'll keep reading from the old schema. See T198308: Enable MCR migration stage "write both, read new" on live systems for the next step.