You can't just "not know" someone one minute, and 'know" them the next. The process of going from not knowing someone to knowing them is called "getting to know someone."
"You'll like her when/once/after you get to know her" and "You'll like her when you know her" woud mean the same thing in most contexts, but the first is better because it communicates your meaning better.
PS: It's better to use full sentences as examples. You've given two dependent clauses, which could change meaning depending on the (missing) main clause.