If I read this correctly you are worried that the SIM used in the locked to AT&T 1520 might not work in an unlocked device???
The lock is on the 1520, not the SIM. The SIM will work in any device capable of connecting to AT&T including a North American Unlocked 950 XL.
If you need additional help or have more questions or details to share, please join the site so you can reply in this thread. See this link
for instructions on how to join MobiFAQ.