Is there a disc to buy which just updates from tiger to leopard which is cheaper ? Im guessing not as I cannot find anywhere that sells it. Looking at the cheaper options of upgrading having spent so much at xmas On ebay there are some leopards update discs going for less than leopard retail, there black discs one says "cpu drop in" and the other says "upgrade dvd". Do these upgrade discs work on any mac? Or does the cpu one only work on the original mac it came with?