Manual:$wgCommentTempTableSchemaMigrationStage

Details
Comment temp tables schema migration stage. Use the SCHEMA_COMPAT_XXX flags.

The default value is an empty array, which is equivalent to:

To perform the migration, set this configuration variable to write to both tables but read from old temp table:

Then, run Manual:migrateRevisionCommentTemp.php to backfill the new table.

Once the script has completed, for testing, this configuration can be changed to still write to both tables but read only from new table. This allows easy rollback to read from the old table in case bugs or incompatibilities are found: