Cripes, why?!
Seriously though, from what I understand there is no direct Betrayal from Freeport to Kelethin. You would need to betray Freeport, be exiled to Haven, complete the quests to become a Qeynosian citizen, then move to Kelethin from there. You can move from Qeynos to Kelethin without "betraying" as they are ally cities. Once you are a citizen of Qeynos you just need to speak to Assistant City Registrar, Gibrien Marsden in GFay who will send you along on a series of faction building tasks which will eventually result in you earning Kelethin citizenship and becoming a unwashed tree-hugging hippy.