This is extremely frustrating. When I get into a PvP match, I usually swap some useless PvE ability for my PvP abilities.

When you're putting normal class skills on your cross bar you just have to press square and it'll let you do it, but for PvP and Cross-class (seeing as you can "unequip" these) you get prompted with a drop down list of "remove" and "assign to cross bar." The "remove" is first so if you go too fast it'll "unequip" the PvP skill, and then you have to re-equip it, and like most cross-class skills, it'll be on CD.

So yea, quick change I thought I'd suggest.