As for me, I strongly recommend do not this until you make ALL backups: mtd0 on NAND flash (256kb) + mtd1(3.8mb) + mtd2 (124mb) + SPI FLASH (64kb).
mtd1,2,3 - is NAND and it could be backed up from openwrt loaded through dnsmasq/tftp
but to backup SPI FLASH needed spi programmer (something like http://pics.livejournal.com/wierus/pic/0002d93b )
It gives you chances to roll-back all changes (IMHO)
Because if you didn’t ALL backups (all!!!) - you may have different issues (including like me ))) ) without any 3rd party help (now I don’t expect help from this official forum - all “it’s up to you” in my hands).
But in my case I only try to upgrade firmware with stock software - netinstall (my story here http://forum.mikrotik.com/t/mikrotik-routerboard-951g-2hnd/85705/1).