It appears the Canadian models also have locked bootloaders which prevent you from downgrading the entire firmware.
I saw the MG1 4.2 modem in this thread: http://forum.xda-developers.com/show....php?t=2324667
Once you flash that it should give you access to the hidden menu. You can then re-flash the MK6 modem after you unlock (also in the above thread).
I have fido on 4.3 stock I337MVLUEMK6 rom. I tried to downgrade it to I337MVLUAMDJ using the Rogers (http://stockroms.net/file/GalaxyS4//SGH-I337M) to get the hidden service menu. I was able to get it downgrade and show the baseband and build version to : I337MVLUAMDJ using ODIN 3.09 and access the service menu(http://forum.xda-developers.com/show....php?t=2285431) to go through the steps and unlock the phone. However I got the invalid sim error and also the phone wifi and sound doesn't work
Any ideas how to get it work on the I337MVLUAMDJ firmware?