For years I have had a small square book entitled "Boeing Jetliners" which covers each model through the 777 in some detail. I had been staring at an old photo of a silhouetted BA 757 for ages, thinking that something seemed "off" about it. It finally dawned on me just the other day. The nacelles are completely unlike the smooth, one piece Rolls Royce-specific nacelles we see even on the BA aircraft today. Instead they were much more like the nacelles on the L1011, 742 and 743.
My question is why were early 757s delivered like this, only to be converted to the current nacelle later? Why did 757s start coming off the line in the newer nacelle so soon after introduction. Did the change in nacelle mean any internal changes to the engine? What was the advantage?