1- you keep shared hosting open as long as you want before cancelling. In fact, some people keep both plans.
2- your stuff gets copied to the VPS for you. You may need to make adjustments and tweaks because the environment is different. The move is easy, but not hands off entirely. A wordpress that was optimized for shared may need some tweaking due to the new environment.
3- when you cancel the shared hosting portion of your account, you get a pro-rated credit to your account for the unused days.