I think the reason they replaced the “Bring my shuttle” line with “Alert my Star Destroyer to prepare for my arrival” was because the length of the shot of Vader walking through Cloud City meant that there was a notably long pause between when he finished speaking and the cut to the next shot. They did a fine job filling it in with John Williams’ score in my humble opinion, but I can see why they wanted to replace the line with something that filled out the sound mix a bit better, albeit one that seems tonally inconsistent with Vader’s seething tone of voice in the duel sequence.