No, the programming is not changed "once a year" or on any sort of regular basis. It is always in flux--new shows are being prepared (at a theatre in Canada!), new acts are being auditioned. Some of the shows are very little changed since the first voyage of the Magic. Some of the old ones have been retired, some have been changed significantly.
As to the entertainers--one guy in particular changes his show all the time--little changes, but they add up. Another is basically doing the same show as 2 years ago. On the repo cruise in August, even the kids knew all his lines.
So...with rebooking exactly a year from now, I would expect (if past cruises are any indication) that you will see a lot of repeats and some new stuff. Of course, this may be a time when you choose to skip the main stage production and do something else with that time. We never have trouble finding something to do, only choosing which thing we'd like best!