Have TM treat a clean install as a new machine (keep the old backup)

I want to do the opposite of this question: Clean Lion install, keeping Time Machine history?

I backed up my computer, erased the disk, and made a clean install of Yosemite. I want to make sure that the old (Mavericks) backup never gets automatically erased; if there isn't enough space on the backup disk I want the backup to fail, not overwrite the old one. How can I be sure that it won't be deleted? Would it be safe to move the Mavericks backup out of Backups.backupdb? Ideally, I'd be able to treat it as a second machine. In the linked question com.apple.TimeMachine.MachineID.plist is mentioned; it would be nice to generate a new one.