If your SIM Card is replaced with a new one, avast! AntiTheft will automatically send an SMS to your friend's number(s) from this new phone number. In this case, kindly send SMS Commands to this new phone number to remote control your stolen phone.
If you want to lock the phone automatically when SIM Card replaced, change AntiTheft behavior (Main menu>AntiTheft>advanced settings>Protection behavior) and then check "Lock phone".