You can put a different SIM in it. The device (in this case the iPad) itself isn't locked to a carrier. The Apple SIM was only an additional convenience in that

You can put a different SIM in it. The device (in this case the iPad) itself isn't locked to a carrier. The Apple SIM was only an additional convenience in that you didn't even have to put in a new SIM in it (i.e. the same SIM could be used on multiple carriers) to change carriers. However, that's currently effectively limited to 3 carriers now (2 US and 1 UK) with AT&T effectively making it non - reprogrammable SIM if activated on AT&T).