For a property being auctioned, you should write to the Agent before the auction asking if their client/s would accept bids from you at the auction on the basis that your requested changes to the contract are made if you are the successful bidders.
If the Agent agrees, that’s good. If they don’t, you’ll have to decide whether or not you are willing to bid with the contract unchanged. If the Agent ignores your email, you should take a copy of it to the auction and if you are the successful bidder, you can insist that your requested changes are made to the contract on the basis that the auctioneer accepted bids from you.
If the property is passed in to you as the highest bidder, then you can immediately ask that your requested changes to the contract are made, i.e. everything’s negotiable after the auction has failed.