Thx for your hint. I’ve spent already a few hours to migrate and adapt everything again. This time it worked without disconnected nodes.
Anyway I had the old merging project still on my HD and gave the renaming a try just to test if this could fix similar issues in the future. Unfortunately it did not work (but made it worse). The “OnActorUsed” is defined in an Interface which is integrated by the Useable_Actor which is the ParentClass of World_Actor. After renaming it in the Interface (compile and save, fix redirects and such stuff just to get sure) the Useable_Actor got the new name but after Editor restart the World_Actor still had the old name (even if it is a child). I’ve got disconnected wires again and even worse the call to the parent function is gone now. And it’s greyed out so I could not add it anymore (which makes sense I guess because that function does not fit to the parent with this name). So it’s pretty messed up now 
- However as I wrote already, I finished a further migration with success (and based on this one I made a further one into anoter project just to get sure everything sticks together). So there is nothing lost (except the trust into BPs and a few hours work). It’s a pain to keep a project clean with BPs. The “fix redirections in folder” function is more a hope(less) function. Deleted folders come back and redirectors stop working. Answerhub seems to be no sollution for this issue as well as I found various threads with “did it work with a clean project”? Of course… “hello_scary_blueprint_world” would not show this kind of issues ;).