virtualbox.org

okay, but is in enough that the old vdi is mounted in Vbox, and then I can open it as a drive from the new Ubuntu OR
I need to copy it (like normal files) first into the new Ubuntu 15.10 and mount it from there as a new drive? I would have then the old vdi in the new vdi ?

I am really thankful you for helping me on this.

You're welcome. My typing skills are such that it would take me all day to explain in detail exactly what to do. It depends on whether you are keeping your Kubuntu guest or not.

i care just for some files files my old kubuntu vdi, dint care about the system files. I will use my new Ubuntu after I get those.

k, then with ubuntu and kubuntu shutdown, you can just attach the old .vdi to your new Ubuntu guest as a second drive. Start Ubuntu, Copy the files you want over. Shutdown Ubuntu, detach the old .vdi.

Then you can just remove Kubuntu from VirtualBox and choose "Remove Files".