If I buy a new mac that comes with a free upgrade to Lion, can I use the upgrade on a different machine instead of the new machine, or is it tied to the new machine? The upgrade site requires the serial number of the new machine. I prefer to have Lion on the old machine and Snow Leopard on the new one, then maybe I will buy Lion for the new machine if I decide I like it.
It is tied to the new mac. The store verifies the mac rather than the Apple ID to allow the download if you need to reinstall the OS.
Some people are reporting that the few macs that shipped with Snow Leopard from the factory, but were changed later get a redemption code that works more like you purchasing an upgrade. That purchase seems to work on 5 macs like the other App Store purchases.
Not 100% sure, but I think that when you buy a new Mac with Lion, is not like if you bought Lion through the Mac App Store. So you'll have to buy Lion for your old machine.
And most importantly, some people reported that Snow Leopard doesn't work that well on machine with Lion pre-installed, so you might not even be able to install Snow Leopard on your new machine.
Any OS X disc can be used on any compatible mac and has no form of serial numbers or registrations to worry about. I'm not sure new Macs with lion come with discs though, you should check that out online.