Hi there!
If we want to do an schema upgrade the actions for to do there is:
Login with an schema admin to the fsmo schema role holder (you can know doing a «query fsmo roles» in a cmd)
Do a backup of System State
Go to cmd and run the command repadmin /replsum and check for some error.
Go to cmd and run the command repadmin /options serverhostname +DISABLE_OUTBOUND_REPL
Go to cmd and run the commandrepadmin /options serverhostname +DISABLE_INBOUND_REPL
Upgrade schema.
Review logs
Go to cmd and run the command repadmin /options serverhostname -DISABLE_INBOUND_REPL
Go to cmd and run the command repadmin /options serverhostname -DISABLE_OUTBOUND_REPL
Force to replicate from the DC schema holder to the others dc.
Regards!