I was reading about this upcoming milestone last week when attempting to unlock my old Sprint iPhone 5 for use on other domestic carriers. The consensus then was that it would apply only to Sprint devices sold after this date; an old Sprint iPhone 5 will continue to NOT work on other networks.
As far as I know, that is an artifact of the hardware. The network compatibility of early (all?) iPhone 5s was divided across three models. Some can work on other networks, but not in other networks' LTE bands. Depends on your particular model and which network you're trying to move to.
My Sprint iPhone 4S worked fine on CDMA networks in the US, and it worked fine while traveling internationally on GSM / non-CDMA networks.
There's something going on in the baseband firmware that rejects domestic carrier SIM cards, but still allows international carriers when an international unlock is granted.
