There’s a new mod for the HTC One M8 that allows you to swap between any radio firmware versions you want. While I believe some hardware limitations may apply on the CDMA variants, you should be able to flash carrier-specific radio firmware between any GSM carriers with no problem.
The mod requires S-OFF and for you to have used SuperCID at some point to set the CID to 11111111, but after that’s accomplished you can flash whatever you want.
The mod doesn’t contain the radio firmware for most carriers, so you’ll need to find it and place it in the recovery-flashable zip file, but after that just flash away and ignore the 10 seconds or so when the thing appears to be completely locked up.
Before you go flashing other carrier’s radios however, make sure you have a good copy of your radio.img somewhere, or a good RUU for your carrier, as you might leave your phone in a completely unusable state.
There are no posts I can locate that contain the radios for all the carrier versions, but if you locate one of the 1.4GB system dumps from any carrier, there should be a radio.img file located somewhere in the massive archive.
It should also be noted that it’s probably against your contract, may be illegal in some states, could brick your phone, and could potentially kill you and everyone you love. But it’s probably not going to.