FRYVA,
I know two people with current 320 ratings, and no Boeing experience, whom have recently started on the 777. It does seem rather hard to predict what fleet you will be offered and seems very dependent on needs at that time (/ day!). This assumes long haul suitability demonstrated in the sim of course.
I'm sure someone will have a contrary story, but I have been told that offers have only ever been changed on a very small number of occasions and this has always been from one LH fleet to another. If you're not interested in SH, personally, I'd not worry about the 'bait and switch' you describe.
Good luck.