Not sure how hard it is, but I had the trim on my Z4 replaced twice by the dealer under warranty. It took three trips to the dealer and a total of about 10 days without my car.
On the first visit they called me the next morning to tell me that they didn't have the part in stock. (so much for appointments)
On the second visit, the trim was replaced. It took a couple of days and they did a craptastic job. I sent it back for a do-over the following week.
On the third visit, they ended up breaking my windshield while removing the trim replaced on the previous visit. This left me without my car for about a week. On the upside, there was much apologizing and a nice 535 loaner.
I'm not sure if this means that the trim replacement is difficult or if my dealer just had incompetent window people. Do you have a trusted place that does window glass repair? If so, you might see if they will do it for less and you can order the trim yourself.