Before fully switching to the new schema, we should exercise the new retrieval code. This means setting
$wgMultiContentRevisionSchemaMigrationStage = SCHEMA_COMPAT_WRITE_BOTH | SCHEMA_COMPAT_READ_NEW;
This keeps writing to the old schema as well, so we can still easily back out of the migration.
NOTE: **This enables the usage of extra slots on the live systems!**
If we are forced to roll this back, we will lose access to the content of the new, extra slots. Rolling forward again would restore that access for old revisions, but extra cleanup would be needed to then again associate that content with newer revisions, made while the change was rolled back.