After you've made a JoeLister listing, you can edit any attributes of the listing directly through eBay's interface. Note that the changes won't be reflected on your "eBay Listings" tab, but they will affect your actual eBay listing. JoeLister will still revise the quantity and handle fulfillment for that item.
More specifically: JoeLister tracks eBay items for quantity update and fulfillment based on their eBay Item ID (the 12 digit number in the URL of the eBay listing). As long as that number stays the same, you can change any other aspects of the listing.