Update old listings after introducing a new FORM

I’m playing around with VANTAGE, and I find it quite adequate for sure to do what I require.

What I cannot figure out thus far though is how to update old listings when I add a new FORM that is relevant to that listing.

All the new listings for that category will have the form filled out so their listing displays all that relevant data;

The old listings were added before the form was introduced for that category; they don’t have that information, I’d like to update the old listings.

Hope this makes sense; look forward to getting a response. I am CERTAIN the answer is right under my nose…