how to connect a new Mac to an existing Time Machine backup

Hi,

a couple of weeks ago, I replaced my old iMac with a new one.

I used the Time Machine backup of the old Mac to fill the new Mac, probably with the Migration Assistant (I booted the new Mac with the USB disk attached and pressed Option; the system asked me if I wanted to restore the Mac using the Time Machine backup - yes, please! Was this the Migration Assistant??). Great stuff, so far (Windows, eat your heart out ;-) ).

But... now my Time Machine backup still thinks it is the backup of the old Mac (that I don't have anymore), and I want TM to recognize the new Mac. I found this link:

http://hints.macworld.com/article.php?story=20080128003716101

but it seems that in Lion, the TM architecture has changed, because, for example, I cannot find files with names similar to .00f9e8d7c6b56 at the root folder of my TM drive (I did use ls -la in the terminal). Besides that, the terminal command for showing the MAC address shows 20 2-digit numbers instead of a regular mac address... weird.

Does anyone know how to do the trick?? My disk is too full in order to create a completely new backup, and I don't like the thought of backing up 500+ GB via the USB cable anyway... I only want to backup the most recent changes.

Just to clarify: Time Machine does see the USB disk with the Time Machine backup as a possible backup dirve, but it doesn't see it as the backup of the new Mac.

Thnx!
Jaap