I wanted to boot up a super old Win7 vm I'd made a ridiculous number of revs back four desktops ago. No VBOX files, no anything - just the VDI itself. Unsurprisingly it wouldn't boot, release, renew, add, or remove from Virtual Media Manager. BUT! Its booted as we speak. How? This one is actually simple:
VBoxManage clonehd Old.vdi New.vdi
chown user:group New.vdi
Then simply add it into your Virtual Machine Manager and attach it to a new VM.
Looks like I haven't powered on this vm since 2012. Its updating now :)