How do we switchover a slave to another master instead of promoting the slave to be the master?

We have a replication configuration wherein there are 2 Masters and 20 Replicas(Each master has 10 replicas). Now, we need to perform MySQL version upgrade on these instances. The approach for this would be to upgrade the replicas first followed by master. The concerns is while upgrading one of the master server(Lets say Master1), how do we switch the replicas pointing to Master 1 to Master 2. Promoting the slave to master wouldn't be a good approach as we have multiple replicas. I read an article about Switching Sources During Failover wherein they suggested : 1. Stop Slave, Reset Master 2. Stop Slave, Change Master to .. However, I am unable to get details/info as to how should we proceed with upgrades in our replication topology and what steps should we follow to do a seamless switchover of replicas.