Q: restore older os from backup to new mac

I have time machine backups of a Mtn Lion iMac,

and I want to move it to a new iMac, but I need to keep the old Mtn Lion OS.

If I set up the new iMac restoring from the backup, will it change the preinstalled newer OS to the older OS on the backup?

(FYI, I have several reasons for needing the older OS, so I dont want to start a debate about that)

If doing that wont restore the older OS, how can I change the new OS on a new machine to the older OS I need?