If you posted extra items and left them running, you can edit them at any time until within 12 hours of their closing. There will not be any additional listing fees, regardless of the price change. That is, unless you add upgrades like gallery. Then you would be charged for those.
Example: I have two copies of a book. I listed both copies in separate auctions during the 10-cent day. I went in and changed the price on one of the books to $300. On the last of the revision pages, just before I had to cancel or submit the revision, it said 'there are no additional fees'.
If tomorrow I decide to sell one of my nice antiques, I can revise the whole auction from a book to an antique...change everything about the listing...and I will not incur any additional fees unless I choose new upgrades that weren't initally part of the auction.