Once the build week has been confirmed, if you alter the order you will go back a stage i.e. the beginning

Something to do with, once the build week has been confirmed, all the parts / options are then winging there way to the production line. So any change will either mean a part maybe not there in time which theoretically could hold up the production line, which there is no way they would do or if an extra is taken out, it means parts lying about!!
That's how my dealer explained it to me when I tried to unsuccessfully add something to my MK6. Being a disbeliever

I then phoned Milton Keynes customer service 3 different times and was told once it had went to build week, no changes could take place. I even pleaded with one and he "apparently" phoned customer services in Wolfsburg but it was still no go. I had only just that day been allocated a build date as well.