Thanks for your replies, but nobody has real experience?

My worry is that while "almost" identical, the N800 and N810 do have differences, such as:
- light sensor
- built in GPS
- HW keyboard

I'm pretty sure there are some system files that are different or settings files have different settings. For example, if N800 backup was restored on N810, it may disable HW keyboard?

You're correct that worst that can happen is a need to reflash the OS... which should not take more than 5 minutes.