Approvals are not transferable simply because you get approved on a SINGLE car that you have chosen, obviously if you decide to change dealerships that car will no longer be the one you end up buying.. Pre-approval by the dealership is also just complete bullcrap, they will 99.9% of the time say 'sir/madam you have been pre-approved, now we just have to wait and see what BMW financial services says'.. But in all honesty I have noticed BMWFS very rarely declines an application, hell I got approved a month ago with 'no job' although I did have an extremely strong co-signer.
Good luck