Re: Can you change it? And does it really matter?
Looking at the somewhat limited specification (http://www.apple.com/ipad-air-2/wireless/), it talks about short-term contracts when travelling, and lists UK and US networks.
As per earlier comments, it would seem ludicrous (and potentially anti-competitive) to block use of other networks at a software level.
So I suspect that the actual iPad Air 2 will still have a slot for a physical SIM, and that the Apple SIM is (at least for the moment) an additional software option offering pre-configured SIM services, useful when you are e.g. travelling, or want cellular services on a device when you don't have a physical SIM.
So when you're visiting the US and want cellular access on your iPad Air 2 which has got a UK SIM card in it, you just go to the config page, get presented with the Apple SIM options, make a selection (e.g. based on signal strength where you are located), make payment via the iTunes store, and that's it - job done.
Apple have made the sale, take their cut from the network operator, and you have the network access you need.
When you get back home, your device reverts to using the network which its physical SIM card is configured for.
No competition problems since this is just a convenient additional service for users and doesn't impose any additional limitations upon them, and in fact can be argued to provide them with a significant benefit.
Obviously, this is just the beginning and clearly long-term things will develop. However, so long as a physical SIM slot still exists in the device then there's no issue.
I would be bothered about the contracts between the network operators and Apple - the network operators will want to be able to provide lower cost services when accessed through a physical SIM, but if Apple have put a "most favoured nation" type clause in the contract for provision of services via the Apple SIM then, effectively, the network operators who sign up will be signing their own death warrants. Then again, maybe if the contracts through Apple are truly short-term or require the presence of a physical SIM in the device with a "home country" type feature locked to that physical SIM (the short-term Apple SIM contracts only being available for other countries) then that might keep network operators happy. We shall see...