Did they specifically replace it, or send it out for service or a refurb swap?
I had a problem with my CD player on my 95 Suburban when it was 3 months old and it was "replaced" twice. After I questioned them on it, they said that it was sent to a warranty repair shop and actually swapped with a refurb per GM policy.
I sent a nasty gram to the dealer and GM regional office. I got a NEW CD player a week later and that solved the problem.
I don't know what BMW's policy is but you might ask the dealer......