Abolishing in-restaurant decisions and making guests pre-decide will save a lot of time in the sit-down restaurants as instead of the server having to give recommendations or asking if people know what they'd like, they can just confirm what the guest ordered and put it in the system immediately once they're seated, so a sit down meal should take far less time and they can get a higher turnover of guests.
Also the kitchens will know exactly what to make which should make service smoother. The only issue is sometimes people buy things, like an extra dessert, that they didn't know they would have a craving for 30 days before the meal, and I can't imagine Disney turning down the revenue so I imagine they will let people add extra items if they have the stock, but you won't be able to change your entree choice on the day except under exceptional circumstances.
They probably will let you do so, possibly by paying a 'change charge', up to 24 hours before though, as they do with ADRs now.
I think pre-choosing at quick service will be compulsory only in the popular locations, so that guest who don't want to pre-book their choices still have some options and they get the best of both worlds. But the days of having complete choice at the point of hunger what you want to eat are on their way out I think, in fact you could say it's been like that since the Dining Plan was introduced.
