Can I upgrade the carrier settings without the original SIM?
Yes. Carrier settings are sent when you connect to their respective cellular service. So if you purchased an unlocked iPhone 4 from Apple, and inserted a SIM from AT&amp;T, you would be pushed the latest carrier update. If the next day, you removed the AT&amp;T SIM and inserted one from Bell Mobility, then you would be pushed a carrier update for that respective carrier.

The updates are based on your iOS version number and the wireless network you are connected to (of course you need the respective SIM as well). You won't receive updates should you remove the SIM (or fail to insert one), but then it becomes a moot point because you need the card to connect to a network, making a carrier update rather pointless to begin with.

When the original carrier unlocks the device, it will be able to handle any wireless network. And the new carrier you sign with, will issue you a SIM.