changes on multimaster replication and activate GTID I've been reading a lot about the changes I planned to do to our DB landscape
but I need some confirmation or recommendation to be sure the work I'll do goes without causing a disaster.
Here is what we have on all hosts:

debian8
MySQL server version: 5.7.16-log MySQL Community Server (GPL)

Replication scene all anonymous:
Master               Slave

S3 some DB's       > S4 on channel1
S2 one 150GB DB    > S4 on channel2
S2 one 150GB DB    > S5 one only channel

All replications work well and are max a few sec. behind master. The replicated DBs I use to do backups.
The 150GB DB on S2 is HA and heavily accessed, but it's possible to shut down for let's say &lt; 10min. S3 DB's are not that critical but a &lt; 10min downtime is highly appreciated.
The bigger picture is, S2 is an old machine and needs to go off grid. I like to have the 150GB DB on S4 and S5 in circular replication in order to do load balancing, failover and backups. S3 DB's needs to be replicated to one of S4 or S5 to do backups.
While that changes I like to change all replications to GTID. That change to GTID is the point I'm worried about.
Here is how I think I have to do all that:

on S2 take the services off that access the DB

take all S2 - S5 read only  SET @@global.read_only = ON;

show slave status on S4 and S5
Q: what do I have to watch for before I shutdown the server?

Shutdown all servers

change Mysqld section in config file
S2, S3, S4, S5
gtid-mode = ON
S4, S5
log-bin   = mysql-bin
server-id       = xx

Start S3

Start S4 with --skip-slave-start

Start S5 with --skip-slave-start

try to connect with replication user across all machines as needed

S4 and S5 Change master to ... the corresponding machines with
corresponding user, password, channel and
MASTER_AUTO_POSITION = 1;


Start Slave for channel1  on S4

Start Slave for channel2  on S4

watch logfiles

Start Slave for channel1  on S5

if logfiles doesn't show any errors
restart all servers

make the services that use DB's point to new Servers


I hope I got all together. Any recommendations highly appreciated.
OK, for anybody reading this in future.

I did the first step changing all server to use GTID based replication and it worked perfect.

I did it the way as I wrote in the original post.

Right now I have same landscape as before, just using GTID.

This night I'll go to activate circular replication between S4 and S5 
and if all goes well shutdown S2

I keep posting for the world after.