Changing Boot Drives and Keep Your Time Machine History

Question

A problem with Time Machine is that it is specific to the target and backup drives.  Change one or the other and TM will want to do a new full backup. If you don't have enough space, it will delete your old backups. HOWEVER, this is a way to install a new boot (or other) drive that TM is backing up and get it to pick up where it left off doing incremental backups.  This is extremely helpful if you change your boot drive.  In my case, I moved from a HDD to an SSD but wanted to keep my history intact.  TIme Machine "knows" which drive to backup based on the UUID (unique drive identifier) so merely changing the name of the drive in the Finder won't do it.  You need to change the UUID in the latest backup of TM.
Note: This is not for the faint of heart or technically challenged. It also requires a mild knowledge of unix or at least the ability to follow instructions TO THE LETTER!  You will change the UUID from the Latest Backup in Time Machine from the "old" drive to the "new" drive thus tricking TM into thinking the new drive is the old drive and it will continue to do incremental backups of the new drive.  The technique described below was originally developed under OS 10.5 (and posted on the MAC OS HINTS 10.5 blog but I have just used it under Snow Leopard.  It might work for newer OSs but I haven't tried them.  I actually did 2 things: 1) Installed a larger TM drive and 2) installed a SSD as a boot drive.  I will post under a different title to provide information on installing a larger TM drive since putting that information here may be confusing. 
You will need 2 Unix commands: xattr and fsaclctl.  xattr is included in OS 10.6 Unix shell but fsaclctl is not.  You will have to get fsaclctl from a version of OS 10.5 using the techniques described in the article posted below.  I had an old 10.5 disc and found it in: /sbin/fsaclctl at the root directory of my 10.5 hard drive.  I also performed the technicque described below logged in as root but this may merely logged in as an administrotor. 
** How to change your boot (or any other) drive and get Time Machine to just pick up where it left off doing incremental backups and not do a full backup of the new drive:
1) Turn OFF Time Machine
2) Install the new drive
3) Clone the old drive to the new drive (in my case HDD -> SSD) using SuperDuper, CarbonCopyCloner or even Disk Utility
4) Make sure you have both xattr and fsaclctl commands installed
5) Open Terminal
6) Follow the instructions here: 
http://hints.macworld.com/article.php?story=20090213071015789&query=time+machine +new+disc
Note: you may need to use the command switch "l" (the letter l) to see the ascii version of the UUID
sudo xattr -l com.apple.backupd.SnapshotVolumeUUID my_partition
Note: Unix uses spaces (white space) as delimiters and is always case sensitive so if you have spaces in the name of your drives, you MUST put the drive name in quotation marks:
if the drive name is: My HDD then to list the contents of the drive in Unix would use the command ls thus:
> ls "My HDD"
will list the contents of My HDD
(the command:  > ls My HDD (without quotation marks) will produce an error - no such file or directory)
When you've followed the above procedure and turned Time Machine back on, the first time TM may warn you that the disc has been changed and "someone may be trying to trick your computer" into using the new drive.  Answer: do the backup
I wish I could take credit for this brilliant technique but I just followed it and it worked for me. I also read the entire thread before starting.

Answer 1

The two drives are handled as separate drives, even if they have the same name.
In essence, the old backups are from a drive that's no longer connected; see #E3 in the Time Machine - Troubleshooting *User Tip,* also at the top of this forum.