I would just re-install the VB client OSes and use the old vdi files.
They have changed the way it works but the image files will be intact. You
may have to search for them with a file manager because they changed
locations. They used to be in a hidden folder but are now in VirtualBox
VMs. Remember to show your hidden folders first (Ctrl+H for Nautilus and
Alt+. for Dolphin). It has been so long since I used version 3.x that I
can't remember the previous location -- or it could be old age.