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 got 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 DB's 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 < 10min. S3 DB's are not that critical but a < 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.

1. on S2 take the services off that access the DB
2. take all S2 - S5 read only SET @@global.read_only = ON;
3. show slave status on S4 and S5

Q: what do I have to watch for before I shutdown the server?

4. Shutdown all servers

5. change Mysqld section in config file

   S2, S3, S4, S5

   gtid-mode = ON

   S4, S5

   log-bin = mysql-bin

   server-id = xx

6. Start S3

7. Start S4 with --skip-slave-start
8. Start S5 with --skip-slave-start
9. try to connect with replication user across all machines as needed
10. S4 and S5 Change master to ... the corresponding machines with corresponding user, password, channel and MASTER_AUTO_POSITION = 1;
11. Start Slave for channel1 on S4
12. Start Slave for channel2 on S4
13. watch logfiles
14. Start Slave for channel1 on S5
15. if logfiles doesn't show any errors restart all servers
16. make the services that use DB's point to new Servers

I hope I got all together. Any recommendation highly appreciated

Thank's Ju