You must have a script that's physically removing the actor's skills then. Whenever I change classes, I still have all previous learned skills. Unless you want those skills to be removed whenever you change classes and reverted back.
EDIT:
Regardless, it was an easy feat. Whenever you change classes, all previously learned skills will be wiped, new skills will be learned according to the new class. If you choose to revert back, it'll re-learn the skills. Now this is simply going off the assumption that you aren't using any custom scripts or systems to force the actor learn skills.
class Game_Actor < Game_Battler
alias gg_change_class_skills_lat class_id=
def class_id=(id)
gg_change_class_skills_lat(id)
@skills = []
for i in 1..@level
for j in $data_classes[@class_id].learnings
if j.level == i
learn_skill(j.skill_id)
end
end
end
end
end