There's nothing stopping such a person from learning additional skills on their own. There's nothing stopping that person from them asking to prove that they do

There's nothing stopping such a person from learning additional skills on their own. There's nothing stopping that person from them asking to prove that they do have these additional skills. There's nothing stopping that person from them proving that their new skills provide more value than their old skills. And if that doesn't work, and their new skills are truly valuable, then there's a very good chance that somebody else is willing to pay for those skills.