What is the difference of using GetChannelClientId() verses GetDeviceUniqueId() which returns the box SN ?
They are both unique to the box and can't be reset. I don't have many clients but this change causes quite a headache with me and my existing users.
How can I relate the caller to my database of paid users which is keyed by serial number ?

You probably shouldn't have used the unique ID in the first place. There is/was something in docs about it. I believe GetChannelClientId will return the same value from all devices registered to the same account and has the advantage that if a user gets rid of a device (and deregisters it) the new owner can't "inherit" rights from the old owner.

-JT