I doubt FDX will be worried about intermixing the fleet engines. They won't intermix engine manufacturers on the same bird but will do so within the fleet. Rumor has it (RHI) that FDX is planning a uniform flight deck with the same layout across the board. I have heard that they desire a 5 or 6 tube EFIS configuration as opposed to the original layout of the 757. There is a company that does it - but FDX wants to ensure that the company will stay in business all the way through the 8 years of mods we have coming. (ie they don't want to see what happened to the 727 conversions (-217) happen to this company. We'll see if that works...soon enough.
I've heard the first 757's have arrived from US and VARIG.
I've got my bid in....