[Nexus 7 General] mstrk242:
**Disclaimer** If you don't understand what follows, don't attempt this. I'm not responsible for you softbricking your device,

This is a quick guide for how to install the new android without losing anything. You'll need fastboot for this. Also, before doing this, be sure to download the supersu cwm update zip, and have it on your sdcard. Make sure you also already have a custom recovery installed (I did twrp, but i'm sure cwm will work just as well). I would also suggest having a backup done, through titanium backup and through recovery, just incase.

First, download the factory images for the n7 from google's website. Decompress them (if you're in windows, you'll need winrar or something similar.) After you decompress the archive, there'll be another one inside (image-nakasi-jop40c.zip), decompress this one as well. Under here, you'll see the .img files for all the partitions. For now, go up one directory. You'll also see here bootloader-grouper-4.13.img. Once you have all this, reboot into the bootloader.

Once you're in the bootloader, you can update the bootloader to the new version (this step isn't needed, but I did it anyways). Plug into your pc, and type fastboot devices. Make sure it's not blank. If it is, you have to update drivers. If you can see the device, open up a command prompt, and cd to the directory that has the bootloader-grouper file. Type the following:

fastboot flash bootloader-grouper-4.13.img

then, after it's done, type:

fastboot reboot-bootloader

Now, you'll be booted back into the bootloader, it'll say 4.13. On to the rom.

cd into the folder image-nakasi-jop40c (make sure you see the files system.img and boot.img before you continue. If you don't see them, the next steps will do nothing but erase your kernel and system parition.)

now, you see the image files. Type the following (still in the bootloader)

fastboot erase system

fastboot flash system system.img

wait until it completes, then type:

fastboot erase boot

fastboot flash boot boot.img

Once this is done, hit the volume up on the device until it shows recovery mode on the top. When it does, push the power key and you'll boot into your recovery (shouldn't be touched.) Inside here, flash the supersu cwm zip file, which'll flash the superuser binary, and supersu. Once this is done, do a factory reset (removing your data and cache.) Boot into the rom.

Once it's booted, you'll notice your internal sd card appears to be empty (mine did, I was worried at first.) For some odd reason, it moved the entire contents of my sdcard into a folder on it called 0. When you get back into android, simply move the folder all up one level so they're in the proper place. You now have the rom booted, rooted, with all your data. Now, you can do a titanium restore to get all your stuff back.

My first boot got stuck for some reason. if it happens to you, just hold the power button and hard reboot. (I did this on 2 devices, it only happened to one of them, so I figured I'd give you all warning.)

Links:

SuperSu binary: https://docs.google.com/open?id=0B7a8xHNJlpgTR0ZkR1pWZWR2VzA

Google Factory Images: https://developers.google.com/android/nexus/images