Hi Perch,
Would they be changing it in the CMS or on a front-end web form?
If it's in the CMS, you can right a plugin for it. There's a plugin hook called 'record_postsave' that passes you the $oldRecord. So you can check what the previous value was.
If it's in a front-end form that lets the user update the checkbox, you need to check for an existing value before saving the new one, and then you can trigger actions based on that.
Hope that helps!
Dave Edis - Senior Developer
interactivetools.com