If you are like I was, stuck on FRF91 without being able to downgrade to get root, or if you just want to be able to flash to any earlier version Nexus One rom, then this post is for you.
FRF91 does not allow you to flash to just any earlier version rom so for this tutorial, we will be using the EPE76 rom that I have already renamed to passimg.zip which allows us to bypass this limitation. The steps provided in this tutorial are all done on a stock t-mobile Nexus one with a locked bootloader, this is especially useful for people who wish to use the rooting method without unlocking the bootloader. If you already have an unlocked bootloader, there is no reason to follow this guide.
"Update" confirmed to also work on at&t nexus one
Notice: after you do this you will have to redownload your apps.
1. Transfer the provided passimg.zip file to the root of your sd card.
2. Power down your phone.
3. Hold down the Volume Down button as you power your phone back on.
4. A screen should appear showing your phone’s system searching for various files. This will take up to 2 minutes.
5. The phone will ask you if you want to start the update. Press the Volume Up button to start the update.
6. If you followed these steps correctly, Congratulations, you are now on stock EPE76 Android 2.1 and able to flash to any official Nexus One rom that has been released.
If this guide helped you please let people know, the more posts there are, the more comfortable other people will feel to follow this guide.