Everywhere I've looked, I've found that the HTC One physical hardware (USA) is the same regardless of the carrier--all models of the USA phone containing both CDMA and GSM radios.
If this is the case, why can't the Sprint variant of the HTC One be hacked to work properly (see issues with Sprint-variant SIM unlocking here) with USA GSM networks like AT&T? Or use GSM ROMs for that matter if all the USA hardware has both radios.
I ask because I made the mistake of buying a Sprint HTC One on eBay with the intent of unlocking to use with my existing AT&T account. NOTE: I am not talking about 4G/LTE data, only 2G and 3G.