How to attach a virtual hard disk using VBoxManage?

What is the best method for setting the virtual hard drive (VDI) of the primary controller for an existing virtual machine?

Does the syntax change if the VDI is really a child differencing disk of some other parent disk? Do you need to attach the parent VDI and then the child VDI in some way?

Situation:

I have an existing VM --- I want to replace the hard drive it uses to boot - with either another normal virtual HD or possibly a differencing disk. Can this be done with VBoxManage?

From serverfault Adam

• I'm not sure if you can do it though VBoxManage, I've always changed it through the GUI after using CloneHD, you answer may be in the VBoxManage Manuel

  From Mr Shoubs