***I've got Time Machine set up to back up a MacOS Leopard Macbook to an external drive. Because of the way this computer is used, it's usually not plugged into the backup drive.***
Time machine will recommence back ups from the last point backed up.
***What does Time Machine do in this situation? Does it keep track of all the changes since it last saw the backup drive.
Apple updates the Time Machine support documents to reflect only the latest version of the OS.***
It should scan and pick up alterations to previous back ups as a matter of course.
For all matters dealing with Time machine Google 'Pondini' and a variety of options about it will appear.
For trouble shooting ...Time machine. http://pondini.org/TM/Troubleshooting.html