These details seem covered in the text above. I'll explain again below, but please let me know what wasn't clear.
If they are on the latest track, they don't have to do anything. If the installed the 17.06 track, they need to get out of it.
At this point there's no provision for closing tracks, as that would be extremely aggressive and break down anyone that relied on the promise of installing a snap from a given series of versions, which is the whole point of having tracks in the first place. We're planning to allow hiding of tracks, though.
Nothing magic happens. The reason tracks exist at all is to allow people to pick a series of versions and stay with it. We can't offer that as a feature and then magically shift which series is being used.