The wood is sandwiched between the lips on the bedsides and the crossmembers,so removing the bed won't do anything.What I think you are going to find is that somebody cut the plywood slightly wider than the opening,slid it in and pushed it to one side until it dropped into place.Then slid it in the other direction until the lip covered the edge of it.I bet if you knock it to one side you will expose the edge of the plywood.Then just push it up and out.