I just bought an Xperia Z and the bootloader is locked. Do I need to unlock it in order to run it on a new SIM card and can I switch to Virgin? It's OK if I can't: I will just buy a Bell SIM for $10, but will the phone be able to be switched to Bell pay as you go? It was previously on the prev. owner plan with Bell.
The bootloader being locked has nothing to do with whether it's SIM-locked to a particular network. It just means you can't flash a custom ROM to the device (without unlocking the bootloader). You don't need to unlock the bootloader to change SIM to a different network, but if it's SIM-locked to a particular network, you need to unlock it from that network.
Whether you can switch to a PAYG SIM is entirely up to the carrier, and nothing to do with Android. You should check with the carrier.