Re: Not very difficult...
A couple of things.
Seniority is seniority is seniority. A lot of guys went from 727 flight engineer to 737 captain at both United and Delta six or so years ago.
Transitioning from Beech 1900 captain to 727 flight engineer kicked my butt because being an FE was nothing like flying a turboprop. Different procedures and you literally had to have the "Geordie La Forge" knowledge of the aircraft systems. For example, if you need to download 4 AMPS of power from the electrical system, you're supposed to know off the bat which systems you can take offline (turn off) to reduce electrical draw. And do it fast too!
Transitioning from the 727 FE to 737 FO was a piece of cake. I knew jet aircraft systems already and Boeing builds pretty rock solid, easy to understand equipment.
Transitioning from the 737 FO to MD-88 FO was a pain in the butt. Primarily because there is nothing whatsoever common between McDonald Douglas aircraft and Boeing aircraft. Additionally, if the Boeing autopilot spoke "English", the McDonald Douglas autopilot speaks "Drunken UK Cockney" -- you can
sort of understand it a little, but you're never quite sure what it's saying.
A good example:
Boeing: "Walk down three blocks, turn left where you see the dump truck and you've arrived"
McDonald Douglas: "OH yes, verr'well, awright...two aves down, turn'leff by the tipper lorry, and bob's yer uncle. Fo'e quid!"