I don't really think they'd be pumping up Christensen's return if he was played present-day, physical-reality Darth Vader. In that case, it'd basically be an easter egg, since it could be anyone inside that suit, and James Earl Jones (or a soundalike) would be doing the voice. It's certainly possible we'll see Darth Vader qua Darth Vader, but I expect Christensen is going to be involved for flashbacks or visions. Probably least likely is some kind of figurative filmmaking thing, where we're seeing Anakin in Darth Vader's place through Obi-Wan's eyes.