I'm afraid you will have to root and install a custom recovery and ROM in order to get kitkat on the d800 with a non-AT&T service provider. There is plenty of info out there on how to do this but for the d800, you must downgrade your firmware to d80010d in order to root and install a custom recovery. The LG Flash Tool is what you'll need to downgrade. It's really easy to use. I aslo recommend using ioroot25 (sorry no links. I'm on mobile) to root your device and it also has all of your adb interface tools that will be used for installing your custom recovery. Again, sorry for not posting links but it should be fairly easy to find now that you know what to look for.
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---------- Post added at 05:51 AM ---------- Previous post was at 05:44 AM ----------
Almost forgot. Get into you file explorer (I use ES File Explorer) and find your service provider data and back it up. It will be a hidden file so you'll need to enable that option. Its location will be /sdcard/.data/.data -- these are your service settings so that if your service provider settings are lost in this process, you can easily restore it once it has been unlocked again. All you would do is replace the file and reboot. Again, plenty of info out there on this. Hope the vague info doesn't put you off of trying this. I'll try and get some links in here later today when I am able.
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