It is possible if the Motorola MPx300 or Nokia 9300 is unlocked. It is also possible if these phones are locked to T-Mobile. However, in order to switch between carriers, the receiving phone has to be unlocked.
Is it possible to change out a sim card from a sponsored T-mobile phone and put it in a phone that takes sim cards but is not sponsored by T-Mobile. Basically, I want to take a sim card from a T-Mobile phone and put it in either the Motorola MPx300 or Nokia 9300.

Chowhound
Comic Vine
GameFAQs
GameSpot
Giant Bomb
TechRepublic