Hmm, a lot of nuances appear here.
What kind of hero is it in the campaign?
1) Standard hero
2) New hero
3) Modified standard hero

This determines where the file of the hero to be changed is located.
In the first case, there will be no hero file in the map; you need to extract it from data.pak.
In cases 2 and 3, the hero file will be in the map.

Nuance #2 - starting skills may be written in the map, so they need to be changed in the map using the map editor (I might be wrong because I've never made campaigns).

And there is also the factor of scripts - hero specializations may be made with scripts, so to change the specialization, you need to rewrite the scripts.