I personnally see Mythic Paths / Abilities in a different way :
A character is "mythic" because he's the hero, because legends will be told about him for centuries. Like David vs Goliath, Ulysse vs the Cyclop, Jason and the Argonauts, even Batman vs the Joker.
In my opinion, you should'nt be able to "change" you mythic path. It represents what you've achieved, what "defines" you in the legends. Achilles was a warrior. It defines who he was. He was other things, yeah, but his path was "warrior". Gods and the universe itself decided that it was his path.
In terms of "rules", if you pick a path (because you're your character's "god") then you can't erase that without changing the whole legend from the start. Achille was a warrior from his birth, according to the legend.
And the abilities ? Well, when you know how to do something, can you "unknow" it ? Okay, it's a game, if you wanna change your character, why not, but I don't think you can do that without Deus Ex Machina. These abilities come from the gods itself, from the legends written about your character. I see it a little like Ragnarok. It has happened before, as it is happening, as it will happen later. Legends about your hero are being written, but you would not play them if it was not legendary, so you play this story the way it will be written later.
I don't know if I explain correctly my point, as I'm not english, sorry if it's not understandable.