I've changed computers a few times, and reverted to previous versions on occasion.
What I do is keep a Vegas directory with subdirectories for each version of Vegas I own, containing the installation file of each revision, license key, documentation, etc.
Reverting is then just a simple matter of uninstalling the new one and re-installing the old.
I copy all this to new computers and am quickly back up and running with Vegas. (Though I occasionally have to touch bases with Magix to be sure I'm only running on two machines.)
It's not really a rollback. Got to uninstall and then reinstall. For a more thorough uninstall, use Revo which will run the standard Vegas uninstall first and then give the option to search the disk and registry for leftover pieces.