To the best of my knowledge, buyers would have to buy the game again at that increased price. The best approach would be a separate store page with the DLC/Expansion and some links back and forth. If you take that approach the product could also be more easily gifted to others.
I feel like it was Dark Dimension who answered a question very similar to this one at some point in the last 37 days, but I’m too incompetent to find that thread again. (Edit: There you go.)
The last thing about the Sub-products I heard was, that they were still in a testing phase and only available to admins - for said testing. If that feature had gone live they would have probably announced it.