Re: "Teen Champion" to Regular
I've been thinking about this myself. I'm starting a Teen Champions campaign in January, and I let the players know we have the option of continuing the characters into a young adult campaign. The transition should be a little easier for me, since I am allowing power frameworks (if I use the Hero System). I am forbidding only stat and skill inflation, and only at first, and handing out extra XP, ~50% of that going to "base", as well as Specific Awards to represent the course of their schooling. During the Teen phase of the campaign, I expect them to transition fairly smoothly to adult characters.
In your case, I would plan an adventure around their graduation exam going all bullocks into an emergency situation, providing both transformational events and the classic "tempering fire". Ask your players to provide fully mature write-ups ahead of time so you can plan what happens to each in advance.
Yes, I realize I'm contradicting Ghost-Angel's advice against jarring rewrites. That's the beauty of storytelling, RPGs and comics in particular; there really isn't one right way to do it.