This is a non trivial operation. Only proceed if you have adequate rollback mechanisms in place, such as snapshots of your virtual machines and a backup of the database.

ALWAYS UPDATE SLAVES COMPLETELY TO 8.x FIRST - BEFORE UPDATING THE MASTER.

WHEN UPDATING MASTER-MASTER CONFIGURATIONS, STOP THE OTHER MASTER DURING THE UPDATE PROCESS with service mysqld stop.

MAKE SURE THAT BOTH MASTERS ARE 8.x BEFORE RESTARTING BOTH AT THE SAME TIME.