ND,
The "answer" is yes as a WSOp you can swap trade specialties however in reality very few folk ever achieve this.
In theory under the new WSOp system an ALM can re trade to Air Eng or AEop. Whilst I know plenty who have more than enough IQ to make the change the ALM branch is shorter than most Officers arms so it's never going to happen due to numbers. Then when you factor in the extra training costs.........
Conversely several Air Eng's have "seen the light" and in order to serve full term have sought to cross over to ALM but as their trade is desperately short of numbers that again is as rare as rocking horse sh1t
Hope that helps.