heh - I decided to live dangerously and do an in-place 9.2->10 upgrade, just to see what would happen (after a full disk-image backup, natch )
I won't say it was ... painless ... but I've managed to get pretty much everything working as I like.
Fixing/replacing the addons/plugins that were broken in the Python 2->3 transition was the longest part, but it was more a case of step repetition than anything mind bendingly difficult, once I got rid of the defunct dependency packages and updated some repos to their Matrix versions.
The most "technical" parts were probably tracking down why some of my remote button customisations were broken - turns out that Kodi has finally gotten rid of the XBMC. prefix from the various ActivateWindow() functions (A quick search/replace operation in keymap.xml and all was good again.) and tweaking my favourite skin (Ronie's Transparency) to work as it hasn't been updated for Matrix.
Really the only thing that doesn't work as I'd like is the TV-NextAired plugin - the Python 3 version works very differently than it's Python 2 predecessor and has a very different UI layout.
To be honest I'm rather surprised that most of my years worth of endless tweaks and modifications and customisations to OS/Kodi/Skin/addons/settings carried over as well as they did.
I'm an old school user since XBMC on original XBox and have a pretty good idea of how things work under the hood, so your mileage may vary.
I *certainly* wouldn't recommend doing this unless you are like me and just want to experiment - if in ANY DOUBT whatsoever follow chewitt and the rest of the LE team's advice, and do a clean install