Honestly, I think Schobel could do well in a 3-4. He wouldn't have a seamless transition, but I do think he could do it. When Schobel decides not to just rely on his speed around the corner and fine tunes his entire game, he'll be a guy who can thrive in either the 4-3 or the 3-4.
As for the rest of them? Posey is, of course, better suited for the 3-4. Spikes plays like a 3-4 LB now. The difficulty would be finding someone to put inside w/ Fletch. You'd have to have someone who can help in coverage as well as stuff the run. Sam Adams might have the most trouble adjusting, as he'd be forced to take on more double teaming and would wear down even more, which has been his biggest problem the past couple seasons.