After getting my hands on an AT&T HTC One, and reading everyone on XDA who said that a 100% conversion to a Google Play Edition was possible, I was excited to give it a shot.
After spending time over the last few days researching and actively doing the conversion on my own phone, I've come to one conclusion.
A 100% conversion is not possible on any HTC One variant using the current tools that are available. Simple as that. Why, you ask? Sure, you can flash the radio, bootloader, ROM, kernel, and update it via OTA. For some people that's good enough. However, if you unroot the GPE ROM, lock the bootloader and return to S-ON (you know, factory state, as a real GPE comes out of the box), it's clear that your phone is not 100% converted. Here are the problems:
1. The "tampered" flag on the bootloader gets set and there's nothing you can do about it.
2. The 4.2.2 and 4.3 ROMs run fine with S-ON and the bootloader locked. The KitKat ROMs do not, however. Regardless of bootloader lock state, if you have S-ON, your ROM will boot, then immediately reboot over and over ad infinitum.
3. The 4.4 and 4.4.2 bootloaders will not unlock on a converted device. A "fastboot oem unlock" command simply makes the bootloader scream about not being able to find a custom splash screen. This problem does not happen on a real GPE.
4. If the HTC One you're wanting to convert is SIM locked, and you plan to unlock it, make sure you do so BEFORE you do the conversion. SIM unlocking on the HTC One is baseband and/or ROM dependent. So, for instance, if you have an AT&T HTC One, the SIM unlock code will only work if you have an AT&T baseband flashed and (possibly) are running the AT&T ROM. Your unlock code will not work using the GPE ROM and GPE baseband.
5. None of the problems mentioned above are related to having the wrong MID or CID.
As for the reboot issue, running any KitKat ROM, custom or official GPE, on any non-GPE HTC One variant will cause that problem if the device has S-ON. It's a well known problem.
The fact that a real HTC One GPE runs its official KitKat ROM with no problems when S-ON, but no other variant will, obviously means that something about the real GPE is different from converted ones. It's possible that if we had an actual factory-signed RUU to flash, that a 100% conversion might be possible. But since we don't, 99% conversion is going to have to be good enough.
In closing, as long as you maintain S-OFF on your bootloader, you'll be just fine with your "converted" phone, regardless of bootloader lock state. It just goes to show, though, that the old saying is true. There's no replacement for the real thing.
I think I might sell this AT&T HTC One and buy a GPE for posterity while they're still available. With the launch of the M8 coming within the next month or so, I don't imagine the One GPE will be available new from the Play Store for every much longer. Then again, I do kinda like Sense...