How to preserve Time Machine backups and start over with a new one?
The simplest way is for you to note the date and time of your last backup in Time Machine on OS X Mountain Lion. You can get back to it at any point in time in the future just by restoring to the backup with that date/time (this will remain on the disk as long as you have adequate space so that Time Machine does not delete these older versions).


You can easily find this date and time by navigating into the "Backups.backupdb" folder on the Time Machine disk and looking at the timestamp for the "Latest" folder (it is actually a link - so you can even open it to see the latest backup folder name with the date and time).




If you do not wish to note down the date and time of your last backup with OS X Mountain Lion, the next simplest way is to change the computer name either on the Mac on OS X Mavericks before you do your first Time Machine backup on it or on the Time Machine backup disk for the older backup.

In either case, this will make Time Machine create a new folder with the new machine name on the disk and it will do a full backup the first time (so you need adequate disk space to hold almost two copies of your Mac). This way, you can restore from the old or new backup in the future (if you rename the computer name on the Time Machine disk, then you would have to revert that to do a full restore of the old system).


To change your computer name, go to System Preferences > Sharing and edit the Computer Name: text.
OR
To rename the computer name on the Time Machine disk instead, use the following command in Terminal.app:

sudo /System/Library/Extensions/TMSafetyNet.kext/Contents/Helpers/bypass mv /Volumes/&lt;TMDiskName&gt;/Backups.backupdb/&lt;ComputerName&gt; /Volumes/&lt;TMDiskName&gt;/Backups.backupdb/&lt;OSXMountainLionBackupComputerName&gt;


Substitute the following in the above command before running it:


&lt;TMDiskName&gt; is the name of the Time Machine volume name
&lt;ComputerName&gt; is the current computer name (which you will continue using for OS X Mavericks)
&lt;OSXMountainLionBackupComputerName&gt; is the name of the snapshot for you to do a full restore from in the future (you would have to rename it back using a similar command as above before the full restore)