I’ll start this out by saying that this does not work on the CDMA variants of the HTC One M8. This means if you’re on Verizon or Sprint, walk away whistling now. If you’re on most other carriers, you’ve got a device you can easily put the all-Android Google Play Edition on.
You’ll need to be S-OFF, have a computer with an updated adb/fastboot handy, and print out the reversal instructions here, as going from Google Play Edition back to Sense ROMs will take some work and who knows if a thread will be there from one day to the next?
Be aware that everything on the internal SD card is going to be wiped. This is very much like the HTC EVO 4G LTE Android 4.3 update RUU in that it changes the partition layout, and there’s no easy way to do that without just formatting everything. There’s also no easy way to get back to a Sense-based partition without formatting everything, so keep that in mind.
With customs AOSP ROMs available that work and have more features than the GPE right out of the box, the advantages of the GPE seem to be that it will get OS updates slightly faster at the cost of functionality. The GPE edition, like all AOSP ROMs, can’t use the Sense camera, so you’re sort of in for a letdown there as the software processing/dual camera is what really makes up for the megapixel lack on the M8.
If you decide to try this out, make sure you copy your photos and music off of the phone somewhere else and verify they’re good. Don’t trust a message that says “all good,” verify before you destroy your partition layout. I’d additionally make a nandroid and copy it to the external SD card if you’ve got one, or to a computer if available. Also make sure you have a copy of the recovery you want to use handy when you’re coming back to Sense as that’s going to be mauled by the partition update as well.