I think Dan is correct. Once your add is up and running you can not change it. What you can do is edit and add to it.
You can however cancel an ad early. I think you have to give Ebay some kind of reason for canceling your ad.
I can see Ebay's reasoning on this. They don't want a bunch of disputes with people claiming "he changed the ad" after I made my bid. Especially if your bid turned out to be the winning bid.