There are lots of named NPCs who have their levels and classes rejiggered between editions.
It's also believable to me that people can loose levels during retirement or becoming aged.
If you wanted to make a mechanic for it, it's like trading in your adventuring class levels for non-adventuring levels, a Rogue 5/Fighter 5 'Conan' exchanges his levels in Rogue to take a few levels of 'King' or whatever
