you can have their equipment installed their way, or you can have your own purchased equipment installed your way. if you get lucky you may get a tech who will use ur existing cabling providing its up to code, and probably just replace your modified slimline with the correct dish intended for the receivers.
just dont bark demands at the tech, talk like you know the job better then him, and simply ask him nicely if he can work around the work you already have done and most likely it will be fine providing the work you performed is up to code. moment you start demanding, expect him to go into "house wrap" mode. he does after all have to warrenty the work and could be inspected by a quality specialist.