What an idiot!!

I changed the front pad this weekend only to find out it was the rear that is grinding!!! I think i could have got away with another 10k on the front pads I changed.
What would class as bad enough wear that the disks need changing as well as the rear pads, I understand there is a special VAG tool needed to change the rear pads, any advise or are there any threads with a "how to" on this subject?