They don’t have to reprogram everything. Sure, it will take some work to port AOE1 completely to the AOE2 engine, but it’s definitely paying off. I’m working on an AOE1 total conversion mod for AOE2 and I already ported the UI, all unit/building graphics, the textures, sounds, unit voices, ingame music, unit/building icons and some other things all by myself. If I can do that, then all this should be no problem at all for Forgotten Empires and Tantalus.
Also, as @Zatekusen already said: AOE2DE is in a great shape!
Please read my post about this topic:
Here is a screenshot of the Ascent of Egypt Learning Campaign - 1 remade in AOE2DE:
