No, they should change engine so they'll have the source code and can edit/optimize it to see fit. Also, it doesn't require the purchase of Crysis, which a lot of people didn't buy.
With their own engine they could edit the source as they see fit, add in extra things, and release the game as a standalone or for $$$.
Right, because creating their own engine would be an incredibly easy task and wouldn't take years to complete.