Placeholder: I'll give this a shot in the morning on my 635 and may be the 830 too. Want to try something here and hopefully it'll work.
Edit: Did the registry edit and its currently going through the preparation process on my AT&T 635, looks kinda slow
Edit 2: The process kept on failing (unable to open update) so I had to reset and currently updating to 14129 and will start with that OS
Edit 3: Finally managed to get Update 2 on my AT&T 635 on Cyan firmware
Edit 4: And the biggest LOL moment....I was able to install a non-AT&T Denim firmware on my AT&T Lumia 635. Now I have Denim + Update 2 on my AT&T Lumia 635...finally equalizer! LOL. From the numerous posts on xda-developers, I don't think anyone has managed to do this yet. I guess this is a big break through... (it might not work for RM-983 and RM-940 devices without damaging it).
Edit 5: Just for fun, I impersonated a bunch of Lumia Devices (930, 1520, 640, 830, etc.) and managed to download & install some non-635 approved apps (Glance, Hey Cortana, Lumia Camera Beta, Lumia Camera 5.0, etc.) which I knew would not work and as expected, they did not. The only thing that worked was Lumia Moments and Lumia Camera Classic...its faster than the slow Lumia Camera w/ Denim on the 635.
Edit 6: I was able to flash back to the LTA ROM and now the unlocked, unbranded US variant. Back to Update 2 or may be even 10
Thanks a ton OP for posting this method!!