If you have the space and adequate lighting, I would keep them inside on 18/6 (or whatever veg cycle you currently have them on) for a few more weeks. Don't put them into 12/12 now. Even if you put them out June 21 they still be reveg on you. If I was going to put plants out that were already flowering I'd wait until at least mid July.
Taking clones and putting them on 12/12 like mellowkitty said is a good idea if you keep track of which plants they came off of. That way you won't be wasting time and money on plants that might be males, which I'm guessing is why you are wanting to do it. Most, if not all, veteran outdoor growers have mothers and only put out female clones. A lot of smaller growers and growers who don't have the time or space will use feminized seeds.
Also, I don't believe hardening them off will be necessary either. By then, the nights will be warm enough that temp stress shouldn't be an issue.