I am posting this as a reply to some of the other posters, who seem to have gone through the same issues as I have right now. Strangely enough the "solution" was an easy click, hinted at in the post before mine, but since the post was only a hint and didn't spell out the solution completely, I prefer to write this down.
1. My photos library wouldn't show any content in iMovie. I could see the tab on the left, but in the window next to it, no content would show.
2. After reading the posts in this thread, I looked up the Preferences section in the photos app, and indeed the "Set up as System Library" tab was greyed out. I went through the suggestions made by lawrence, but that didn't change anything on the iMovie side... in the process I figured out WHY the "Set up as System Library" was greyed out: because the active library WAS the system library ALREADY. When opening photos with option click, the choice window confirmed that as well. So there was actually no problem at all with the System Library setting. I was looking at the wrong place (mislead by the posts).
3. I had overread the last post in the thread by Andrewid, stating that he just noticed "Years" tab to the right of "Photos Library"... when reading it first I had thought "so what", but then it dawned on me that the solution to all my problems was actually right there, with only a click. The "years" tab was indeed there and to its right it said "all years", i believe... when clicking on "all years", I suddenly got the choice to choose a year and all my photo library was available (again) ... I say again, because it had been available in iphoto, before upgrading. Alternatively, you could also click on "years" itself, and choose between "My Albums", "Albums", "Years", "Collections', 'Moments', 'Places', 'Faces' and 'Shared'... either way, i finally got access to all my photos library again.
Hope this helps the other posters.