virtualbox.org

I have learned that in order to add space to a drive in a VM, it is necessary to
1) convert VMDK to VDI using VBoxManage.
2) resize VDI to new size

Easy so far. Now I have my original VMDK and a VDI of the same VM.
At this point it would be nice to edit my existing VM settings in VirtualBox to use the VDI instead, but I cannot find any way to do that. If I start the VM, it uses the VMDK with the original size.

So I try to convert the re-sized VDI back to VMDK. First I try directly converting, but it fails because the VMDK exists.
So I delete the VMDK and try again.
Now it fails "....because a hard disk 'H:\VirtualDrives\Ubuntu\Ubuntu_14.vmdk' with UUID {828ff606-fe48-4dd8-89f1-98a6e68454ec} already exists"

I would prefer to convert all my VMs to VDI, if there was a process that works. What am I missing? How do I reconfigure the VM's to use VDI's instead?

The operations you need are in the Virtual Media Manager. You need to release the "old" virtual drive from the vm, remove it from the media manager, then add the "new" drive to the media manager and assign it to the vm.

See Chapter 5.3 of the manual.