Hi,
There is a method to Import Closed Listings only from the Aucctiva Closed Listings page (designate the listing(s) and use the Import button). Once they are imported (takes some time as it's a scheduled thing they do), you'll see them in your Imported Listings folder on the Saved Listings page. As a Saved Listing, you can use the mass change tools on that page, i.e. the Find & Replace and Bulk Edit functions. I'd recommend you runs a few checks and tests with samples from the Import to assure you get the expected results from those functions.
I would take the time to get familiar with the listing editor, use of templates, profiles, and folders, as the time spent learning/planning will definitely payoff in reduced workload for a major migration for a Store. Checkout the tutorials and the FAQ at HELP (top of page).
Hope that advise helps.
Danno