I posted on another thread (search for WR849N) an image for the 849Nv6 that should work on the 840Nv6, just flash it using TFTP. I did the same modifications as @gaspare . In fact, for the 849Nv5 I used the image for the 840Nv5, so it should work both ways.
Please beware that my image does not have LUCI installed, so you may want to compile your own with what you need and install using sysupgrade once you have a running OpenWRT on your router.
S.
PS: Indeed, this Thursday I was buying a 849 and the shop gave a discount because it was sold without the box - they told me that the box had WR840N written on it instead of WR849N, although the sticker said 849, that was why they were selling it discounted.