Installing a new sound system?

Hi all, 1st post:

I'm getting a new CTS tomorow, but so far am holding off on the top audio package with the Boss speakers and all that jazz because of the price, and I was thinking of going for aftermarket products anyway...but the dealer has told me that doing so would mess up the onboard computer and it just, in a nutshell, wouldnt work

Is it possible to get a new deck in? upgrade my cd-changer? get a LCD in there, and all that? or is it a lost cause?


thanks for any replies