Let's think about this without proxy bidding for a moment. If I see a name and the current price is let's say $500 and I think I like it but I am not willing to pay $500 so I'll come back online when it falls to $100. A short while later it has fallen to $200. I could now decide that actually I'm willing to pay $200 and want to snatch it up before someone else does so I go on and click buy now at $200. So now bringing proxy bidding back into this, allowing someone to change their proxy bid is really no different and should be allowed.