How can I use my existing Time Machine drive on the network? I have an external HD that I've been using as a time machine for my MBP.  Here's what I want to do:

attach the drive to my desktop machine
configure time machine on my MBP to use the drive as a time machine through the network
retain my existing backups...

So I want time machine to continue using the same backups it always did when the drive was connected locally.  I do not want to start from scratch.
I know how to set time machine to use a network drive rather than a locally attached drive, but it doesn't recognize the existing backups (it wants to start over).  I'm not afraid of the terminal... how can I get this working?
on edit:
I got a large drive to use during the conversion, but the link below didn't help because there was no hex id to be found on my machine in the second step  :/
You'll need to convert your current local-style Time Machine into a sparsebundle before it will be usable in TM over the network. You'll need some temporary storage somewhere to create your converted backup into during this conversion.

I don't know the exact conversion procedure but here are some Google'd links:
http://www.tidbits.com/Talk/1960
http://jason-townsend.blogspot.com/2008/08/how-to-transfer-local-time-machine.html

Something along the lines of hdiutil convert -format UDSB