WARNING! I spoke to Nokia about it and this obviously voids your warranty so only do it if you don't care I did it myself and it worked, if you break something along the way I will not be responsible! Perform this at your own risk!
That being said the actuall process is very very easy and anyone with a screwdriver, a bit of common sense and a spare half an hour will be able to do this.
So first of all you need to order the spare parts. If you are changing the colour you'll need to order three: the housing, usb cover/flap, sim tray. If you replace with the same colour you dont need a new sim tray as you can use the old one. You also dont need to buy the USB cover as the old one can be taken off and put on the new body(however this requires you pulling it off with quite a bit of force which could result in breaking it or making it fit less firmly in future so i suggest that you do buy the new USB door either way) The process of placing a new one on is way easier than taking it off.
I ordered my parts off ebay from "Headlane Ltd"
here is the link for the cyan battery cover:
Search for other parts simply changing the colour in the name or just type " lumia 800 sim tray" or "lumia 800 Usb door"
Anyway once you've ordered the needed items it's time to watch a youtube giude.
Now this is a FULL-FULL guide on how to completely tear the device apart. If you are only changing the housing then watch the first couple of minutes and the last ones.
Basically you only need these steps:
1. Taking off the screen
2. Taking off the battery cover
3. Taking out the battery
4. Taking out the motherboard
The rest (flash, side buttons, headphone jack, speaker, various antennas etc) stay in the old body and will be supplied in the new one so dont take them out. Although the middle of this video talks about how to do that. But i'll repeat again:
if you are just replacing the housing you dont touch those, it will be supplied in the new one. Basically you'll be able to see it when comparing what your new housing looks like to the one you just took off your phone. When they look the same, stop, FF to the end of the video where its being put back together and start doing that.
I did this myself in less than 20 minutes having watched the video prior to process. I wanted to change my black housing to cyan and did just that.
I've uploaded two images, one is work in progress and the other is finished work!
I'm in a bit of a hurry now otherwise i would make it more detailed, so if there are any questions post them! Plus the video is very very good at explaining this so thumbs up for the maker!
Just wanted to add a couple of things now that i have a bit more time:
1.If you are changing the housing cover only whether it's to the same or different colour the relevant part of the video is up to 4:47 and from 14:27.
2.The things that come supplied with the new housing and therefore should be left in the old one are:
Speaker at the bottom
3g/gsm antenna next to the speaker
Wi-fi + BT antenna on the left
Camera flash (not the camera)
Small magnet that holds the USB cover in place
3.I used my thumbnail to remove the screen and in the whole process left absolutely no scratches/dents on the case or screen. It wasnt too hard to do.
4.In the video, the maker talks about the screw thats under the usb door telling us to leave it in at first as it wouldn't come out, mine fell out straight away when i flipped the case so be careful! Don't lose it! Put it back in when you re-attach the screen.
5.I found taking out the top left part of the screen quite hard using my thumbnail, so i used a screwdriver to push out the metal ring under the usb door (the one in which the screw was in). It's kind of common sense when you see it.
6.I also found it a bit harder than described in the video to take out the metal battery cover after unscrewing 6 screws. Maybe i was being too careful but within two minutes i managed to find the right pressure point and it snapped out easily.
7.Be very careful unscrewing the screws. I almost messed it up on the top right out of the six that hold the metal cover. It was in very tight and i twisted the head a bit withought the actual screw moving and it started to look very bad. Solved it by using a larger T6 screwdriver. Others were fine with T5 just dont rush.
8.I didnt use a magnetic screwdriver and theres no need at all.