while i havn't played the remake yet, i assume they having retconned Jenova's ability to shapeshift
because while looking up something about Sephiroth for Dissidia i came across something about him in remake that would easily be explained by the same reason why no one saw Jenova escape the Shinra Building in the original but they did see Sephiroth
@Memor-X I will just say that I would NOT be surprised if the remake is the version that ties in to KH, and remember that KH has that little, insignificant, difference...