Hi
You are correct you can not change the hostname of a Domain Manager when moving it to new hardware. This being said what you should do is name the new server the same as the old Domain Manager segmented to avoid duplicate hostnames/IP address conflicts and then follow the ITCM green book entitled How to Move the MDB for ITCM 12.5.
Its a great idea to streamline the process by using the backup and restore method/manual method for doing this as described in the Green Book.
I would also advise to run these SQL queries to get the exact hostname and case of the SQl server information so you can use the same information when updating the comstore settings
run these SQL queries vs mdb
select * from ca_settings
select * from ca_agent
select * from ca_n_tier
Note results so you have them for later use.
after restoring mdb to new server with same hostname you do the following to update ITCM
1. change mdb information about domain manager
2. Associate the user and logins for SQL server
3. update the comstore settings on the new domain manager
These are the simplified steps and more is involved depending on the ITCM components you are running . Make sure to carefully read the Green book for more details