how about like an explosion? like glass breaking outward towards the viewer..like the old vid is bustin it's way thru.

the "fragments" can either move with the x-fader movements - so if you have the x-fader in the middle, the fragments with the old vid are stuck in mid-air and continue as u move the fader over.

or

it can just explode as soon as you start crossing over, like a quick cut-in time. or explode as you reach the halfway point. i think as soon as you start crossing might be better, but hey...to each his/her own..