I have an Xperia Play that was originally with Verizon and therefore using CDMA. I want to use it with the Rogers network now.
I've read that the Qualcomm chip inside supports both types of networks by default. Is it possible to change it to GSM?
Not going to happen. Sorry. While MSM8655 supports GSM - Play Verizon's version of radio software makes no use of it. You can not reflash the radio software because the bootloader is locked. And even if you could - the GSM bands are not "phased" in your phone. "Phasing" is a process of calibrating the phone during manufacturing - without it you would get no 3G, at best you could probably get very poor 2G performance if any at all.