Hmmmm....it probably depends on the method you were using, Flat or Calc. I'm guessing Flat, or the USPS prices would simply substitute. If you are using a consistent Flat structure and maybe only a few different prices, there might be a way....
How about setting up a Shipping Profile(s) and use the Find & Replace to substitute. You may have to segregate listing by rate needed before you run the batch. I'd at least try that method with a few test cases.
Oh...and once you've got profiles, the next round of price increase is a snap.
Danno