Didn't want to reply, but I had to, something was just making me.
It's really nice you plan to update your Campaigns for Reforged, honestly it's a plus for Reforged as a whole, and a reason more to buy it (against many not to buy it, sadly)
Iirc, Jeopardy of the Horde should be the easiest to update for Reforged. Iirc it basically uses only default models and skins, so it should look fine. There are even some benefits, like Drek'thar, Samuro, Cairne having their own models in Reforged.
Similar situation for Rowan the Wise, the only thing being Bevan and dwarf footmen/knight. The thing to solve the Bevan problem could be to make him female and use Janella model(and rename him). That could work really well, cause Tara is the only female in the group, and he is too girly anyway. The only problem with it would be the voice.....
Haven't played Ressurection of the Scourge.
However I see a lot of problems with Malfurion's Quest. While there are some benefits, like Tyrande, Cenarius, Magtheridon having their own models by default, I definitely see the problem with soooo many custom models for basically all factions. Complete Highborne tech tree with heroes, some legion units (shivarra with heroes), some naga units, some satyr/nightmare units, even some night elf characters like Faradrella(can I call her Lunara
) or the Guardians. I pesonally don't think that custom models look good in Reforged, they basically look like Betty Boop(cartoon character from the silent era) in the middle of Frozen, and to me at least that doesn't look good. You can try passing High Elves as the Highborne and see how it works, but I don't know.
Again have no clue why I just posted this, but I just had to, something was just making me.