Thx for your support, but to be honest, I'm a total coward when it comes to modding files...
Anyways, I played around with the give_trait cheat, and found this:

give_trait "Methuselah Magnus" Athenaios opt:0
give_trait "Methuselah Magnus" Spartiates

Where opt:0 gets rid of the trait.
Assuming the character was an Athenian, these cheats, used through the "~" console code, should get rid of the Athenaios trait, and replace it with the Spartiates trait. So far it's been working for me.