Im sure I've read somewhere that altering the spec of a preordered car can effect the build week... did the dealer mention this?
You can alter the spec of the car right up till it has a confirmed build week. Once it has this date, the extras and stuff start whining there way to the production line so everything is there for it to be built.
If you want to change the spec once it has the confirmed build date, you basically start again from scratch.
I tried to add something to my car and that's what my dealer told me. Not that I didn't trust them or anything but I phoned Milton Keynes and tried the best of three approach.

First guy told me the same as the dealer.
Second guy thought I could change the spec and phoned me back after apparently phoning Germany but then said the same as the dealer.
Third guy told me I could change the spec but the dealer had to make the changes direct. So I phoned the dealer and he apparently phoned Germany as well but it was a no go. So it was left up to me whether I wanted to go back to the end of the queue again. Needless to say I went without the extra.