It's gonna be quite a lot of additional surplus, and, they already do this. Surplus wine grapes get sold to concentrate makers, however, it pays less than it costs to grow them because concentrate is basically a commodity sweetener competing with sugar, corn syrup, and apple concentrate. Bottled grape juice is a small and shrinking market, and in that market, most people expect Concord flavour (these aren't that), plus a lot of bottled juice is reconstituted from that same commodity concentrate anyway.