Currently merchant updates are being done manually by the genius Monzonaut that is @RachelR each time they are submitted.
There are currently no facilities to support merchants that change their merchant ID (I believe) however I have seen that @kieranmch is working on an updated merchant system.
https://community.monzo.com/t/improving-internal-merchant-tooling/52134
Hopefully we will see some progress made in this area soon, but for now just keep firing your updates via the 
app and the team will get it updated within a few days (usually quicker!)
