I would echo this, this needs to be treated as three separate positions.

1. Provisioning of new MID servers, verification of clustering, capabilities, applications, IP ranges etc. and confirmation they can operate as expected.

2. Transition of workload, as well as ITOM, don't forget Integrations, export sets and LDAP/JDBC data feeds can use MIDs.

3. Monitor the ecc queue for any transactions on the old MID servers to verify that you have migrated all the workloads.

Once you have confirmed the above, I would recommend to stop the old MID servers for a time and ensure nothing breaks before fully decomissioning them. If you have any monitoring set up on the old MID processes themselves then you would obviously want to stop that at the same time.

Hope it goes well!

Richard