I have a 2 machine registration. How do I deregister from one of the machines and apply it to another one?
Nothing special needs doing, just uninstall Opus from the old machine (or delete /dopusdata/dopus.cert, e.g. on Windows 7 it is C:\ProgramData\GPSoftware\Directory Opus\dopus.cert), and install Opus normally on the new machine.
If the old machine is being wiped for sale/recycling then you don't need to worry about it at all.