Well, this is a 747-100 cockpit:

http://upload.wikimedia.org/wikipedia/commons/9/92/747-100_Cockpit.jpg

This on the other hand is 747-300 cockpit:

http://www.outdoorphoto.co.za/gallery/data/524/medium/747-300_horiz_runway_cockpit.jpg

Like we can see there's not all that much of a difference between them, all 747 classic series share nearly identical cockpit layout (apart from some later modifications). Thus it really shouldn't be an issue for CS to model them all, it shouldn't be any more complicated than making different 777 variants.