Now, I'm wondering, should I get involved with it? One thing I'm thinking about is doing up physical abilities for their past selves, to represent that the PCs are inhabiting bodies other than their own. They'd keep their mentally-based skills and Awakened magic, though. With this interpretation, it'd be like their minds were psychically projected into the past.
On the other hand, I could save myself some grief and do it Quantum Leap-style, where the PCs are the PCs, only with the appearances of their past selves. If I go this way, I'd play up the fact that it's really more of an ephemeral journey into the collective unconsciousness, as opposed to some kind of mental time travel.