Inspired by this question: Class skill +3, when you have 1 rank in that skill
OK, I have a fighter with a ranks in (for example) Acrobatics. Then I gain a level and add a Rogue. Now, if I started as a Rogue, or add ranks after the fighter multi-classes, I know he will get the +3 bonus, but if you have the ranks in the skill BEFORE multi-classing and then the skill becomes a class skill do you get the bonus or have to wait until you put more rank(s) into that skill?