Hello,
Is it normal that after an upgrade (from DO 9 to DO 10), all thumbnails are being regenerated ?
This is with x64 versions running on Win7x64. All preferences and settings were successfully imported, but now each time I navigate to a movie / images folder, all thumbs are recreated 
Thanks in advance.
It's possible, but if so then it is only a one-off and the thumbs will be cached again once regenerated.
Another thing that can cause parts (or all in some cases) of the cache to be flushed is changing certain thumbnail or viewer plugin settings that cause the cached thumbnails to be invalidated.
If (re-)generating movie thumbnails takes a long time, you might be able to speed things up by changing the Movie plugin's configuration. (Note that doing so is one of the things that will clear the cache.)
The "skip over initial black frames" option can slow things down quite a lot. Also note that you can turn off the "generate thumbnails" option entirely and Opus will then fallback on the shell's thumbnailer for movie files (i.e. the same as Explorer), and also the shell's thumbnail cache in turn.
Different (versions of) video codecs can also affect thumbnail speed sometimes.