Rob,
The first thing to do is to download the Factory Talk Activation Manager from the rockwell site.
You can then Re-Host which activations you want to move. This means that the activations which are 'locked' to a device are released so that you can 'lock' them to another different device or hard drive.
I haven't done it for a while but I'm sure you need the serial number and the product code when you want to re-register the activations to another device.
As I say you need to download the Factory Talk Activation Manager to rehost the activation and its pretty simple to do. You then just get the activation back as if it where a new one.
Hope this helps
Mark