The projects should still work, though jAbum has undergone enormous changes. You will get a 30 day trial to have a look at the new skins. Work on the Sample Portfolio to see what you like, then backup your projects and albums and then work on a duplicate of your projects. Doing that belt and braces approach means you have nothing to lose.
Leaping over that many versions, I suspect the installer is going to leave a lot of garbage behind, files left over from jAlbum 8 that may cause havoc. In this case, I'd recommend doing a complete uninstall before installing jAlbum 20:
Some skins that were available with jAlbum 8 may no longer be available, or may be incompatible with jAlbum 20. I suggest that you should check the status of your favourite skins.
However, much better skins are now available, so you should consider discarding your old skins.
No, the skins for JAlbum 8 are mostly non-responsive, they don't work very well on mobiles and tablets, and over the years that has become more and more of a nuisance as more people are watching it from a mobile device. I want to upgrade just to use a more modern and more responsive skin.
From what I read here there does not seem to be a conclusive answer, so I will just make a backup of my existing JAlbum en see what happens if I upgrade, if if fails or I get weird result I can still deinstall or rollback to the backup.
...I will just ... see what happens if I upgrade, if if fails or I get weird result...
You're not really "upgrading." You're just doing a fresh install of jAlbum 20. And that's why I strongly suggest cleaning out the old stuff - you need to make sure you're getting a "clean" installation. Keeping the old version backed up somewhere out of the line of fire is fine, but don't leave it sitting there in your program directory - that's just asking for trouble.
There's no reason why a fresh install should fail, or produce weird results. I'm assuming you're not running an obsolete operating system, of course. Something like WinXP is going to be a non-starter, for example.
Ok, so I fully deleted my old version and made a fresh install of jAlbum 20.
Still I could not resist trying to import one of my old projects from version 8.12, and so far that seams to work nicely. I got an error that I did not have the old skin that the project used in my new JAlbum and asked if I wanted to download it, but after cancelling that option it all imported fine and I was able to create a fully functional album from it using the Tiger skin that came with jAlbum 20.