They do have upgrade dvds, but isn't that for the interim period between announcement and shipment (and depletion of the supply of the current stock with the older OS). I never understood that to be independently for sale.
I would guess that it wouldn't make sense for Apple to have an "upgrade" OS, since every release is in effect an upgrade. Unlike PCs, you can't just build your own and dump an Windows OS in from scratch. Because you have to go the Mac route, you already have Apple's OS in your machine anyway.