Copying/migrating a VirtualBox image?

[stan]

I have a production VirtualBox system running on 9.10 with VirtualBox
2.1.4_OSE, and I want to migrate one of the images from there to a
different 9.10 machine. That machine has VirtualBox 3.0.2.

I thought i could just tar up the entire contents of .VirtualBox (given
that all the instances in it were shut down) and untar the resultant
tarball on the new machine, being careful to preserver ownerships, and
permissions. 

But, when I do that, and fire up VirtualBox on the new machine, it says it
needs to convert the images. I let it do that, but now the Virtual machines
cannot see their hard drives.

Any suggestions?

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