It is not possible. Was already asked and answered (maybe even it this thread). Reason is - you only have rootfs part on your SD/MMC card. bootloader,kernel and initfs is still in your flash.

In theory you can extract just the rootfs from the firmware and copy it to card but you'll miss changes in other parts of newer firmware (bootloader,kernel and initfs) and the result may not boot at all. In some special versions it can work (2006.39 and 2006.49 on N770 is sufficiently similar and work) but generally no.