X3Photo Gallery Forums

Let me first explain a few things which might change the way you plan your approach:

Imagevue X 2.8 === Imagevue X2
Imagevue X3 0.31 === Imagevue X3

i use ImagevueX 2.8 - is it possible directly upgrade to 3.21 (X3 Build 0.21)?

Imagevue X2 and Imagevue X3 have virtually NOTHING to do with each other technically and logically, and it's NOT just an upgrade between 2.8 and 3.21.  Imagevue X3 is a modern website system, which should preferably setup SEPARATELY from X2. Read more about this here:
https://forum.photo.gallery/viewtopic.php?t=8754

If you really really really need to UPGRADE from X2 to X3, instead of creating a new modern website from scratch, then you can use the process above. In YOUR case though, it sounds like you are not sure about X3 yet, so why not install and try it in a SEPARATE folder first?

If i don't want to use this Version is it possible to go back to the old Version if i save DB-Dump and Filesystem (without Caches / Thumbs)?

Neither Imagevue X3 or X2 uses database, so NO there is no DB-dump. Why upgrade if you are not sure? Just try X3 first. X3 is explained in details on our website:
https://www.photo.gallery

Compared to X3, Imagevue X2 is old, outdated, slow, doesn't work properly on mobile devices, and not many of your visitors have any reason to enjoy it. Personally, I find it strange that someone would see any reason to continue using X2 instead of X3.