How do I have multiple versions of the same .chm?

Our developers recently were required to make changes to an earlier version of our software (thank goodness for backups). So I'm having to revert to that earlier version, make changes, and create a revised .chm. HOWEVER, I already had completed the upcoming release's .chm and don't want to lose all of those changes. To make matters more complicated, I also have requests for changes to the Help for the future release.

So, how does everyone have multiple versions of the .chm on their system at the same time and keep them separate, yet compile the right files with the right version? I had thought I could just create a new folder with the files that changed for the various versions, but updating them in Word 2007 is not a pretty proposition. I'll then have to import them in and replace the "old" ones. There has to be an easier way! I'm sure other people do this on a regular basis, but this is the first time I've had to go back a version, maintain the next version, and start work on a yet-to-be-released version all at the same time.