OS X Mountain Lion: If you want a new computer to “inherit” your backup history
You can move the backup history from one computer to another, but once the new computer “inherits” the backup history, you won’t be able to use the backup history on the original computer.
You’re prompted to specify whether you want a new computer to inherit a backup if you do any of the following:
Use Migration Assistant to transfer your settings from an old Mac to a new one.
Use Migration Assistant to transfer your settings from a Time Machine backup of an old Mac to a new one.
Clone the disk you use to start your computer, or physically move it from an old Mac to a new Mac.
Replace your computer’s motherboard.
If you are replacing an old Mac with a new one, and want the new one to use the backups from the old computer, click Inherit Backup.
If you plan to continue to use your old computer, click Create New Backup to preserve the old computer’s backup history and start separate backups for the new computer.