Is there any harm in using factory.img?

[alienheartbeat]

If I wanted to do a completely clean upgrade of openwrt from (say) 1.5.8 to 1.5.10 could I use a factory.img?

(of course I know I could use sysupgrade.bin with Restore Default Config
but the openwrt forum tells me "factory" has the full flash layout, with the unused space set clear, erasing the area that will be rootfs_data,
whereas "sysupgrade" has only the kernel+rootfs, and leaves "rootfs_data" intact. So faxctory.img should work.)

I ask because I have just been reading the openwrt documentation on flash layouts etc, and am trying to test my understanding of how all this works.

[alienheartbeat]

Just thought of a possible reason: the "Upgrade Now" button in the gui probably uses the openwrt sysupgrade command documented here:
http://wiki.openwrt.org/doc/howto/generic.sysupgrade
which might work poorly with a .img that overwrites rootfs_data.

One would have to use the tftp method rather than the super-easy gui method.

For info, also found this on openwrt which explains the difference in the images:

sysupgrade.bin images are meant for flashing from an existing Openwrt system, either by using LuCI and the sysupgrade script.
factory.img images are otherwise identical, but a short header has been added to the image enabling the original Netgear OEM firmware and the TFTP recovery mode to accept the images.