In Australia prepaid sims for iPad are common. Eg 12gb or twelve months, whichever comes first.
Anyhow, to change carriers in the past, you always had to swap out the sim which is tied to a particular carrier.
Presumably, an Apple sim allows the user to choose the carrier using a software menu based system. For unsupported carriers, you just swap the carrier's sim in.
I think a physical sim will be required for a long time as too many carriers will not participate. And...