Hi Adam,
Track and Accept Changes is somewhat analogous to the Track Changes feature in Microsoft Word. When you select Track Changes, any changes to the project will be stored until someone selects Accept Changes. Selecting Show Changes will show all of the changes made since Change Tracking was turned on.
Currently, we don't have a mechanism to authorize only certain users to turn change tracking on or off.
It sounds like what you're trying to accomplish might be better handled in your revision control system. If you wanted to baseline your requirements, you could either label or branch your repository at specific points. You could then pull specific baselines from the repository. A nice way to show differences between baselines is to generate a Word report for each and use the Compare feature in Word - it will show you markup with the differences.
Let me know if this helps. Thanks!