Congratulations, Bernard. It's exciting to get a new camera with significantly better features, isn't it!
I recall when I got my first full-frame DSLR, the Canon 5D. I was so excited I decided to visit Nepal again to do some trekking, for the first time in 40 years. During my first visit I had a Pentax Spotmatic; during the second visit a Canon 5D. What a difference!
However, to get things in perspective, I don't see much difference in basic image quality between the D850 and D810. From the comparison images I've seen so far, my impression is that the D850 has very marginally better resolution, and very marginally lower noise at high ISO. I was hoping that image quality would be significantly better at high ISO, but I've seen no evidence of this so far.
Any improvement is better than none. However, it's the other features which are the main attraction of the D850, such as higher frame rate, more accurate focusing (hopefully) and 4K video in particular (for me).
I've never bothered much with the HD video capability of my DSLRs, but 4K video capability could inspire me to begin using the video feature and start experimenting with time-lapse video. Eventually, I expect I'll buy a 4K OLED TV, when the really large screens become more affordable.