Assigning Product Inventory to Store Page after product is in inventory

Hoping someone can help me with this one. I've searched for nearly 2 hours and can't find the answer I'm looking for.

I inherited a site that is poorly organized and doesn't use the many features around category assignments. I'm trying to reorganize, and use categories to align the products better. What I can't figure out, is how to assign an already existing product in the inventory area to a store page without creating a new product listing from scratch. Trying to avoid making duplicates and wasting my time.

I can see how to assign a store page to a product when creating the initial product listing, but not how to edit 'Where' the product has been assigned initially. I can also see how to add a new product within the store page, but I'm not wanting to create duplicates of products.

The site is currently using the template AVENUE on SQSP 7.0.