If you've uploaded an item to the new store, you can now link it to the equivalent item in the old store.
This will also import all reviews.
This allows anyone who purchased your old asset to be able to receive updates via the new asset, and is only suitable for assets where they are for all intents and purposes equivalent.
To do this, edit your item, and on the left menu is a menu link:
"Link to Scirra item"