We don’t really have a good solution for this. Can you describe in more detail how it works in the Unity Asset store (or link me to an example project so I can take a closer look)?
The only approach I can think of for here is to create a brand new product page for the new update, and put the old one in “Restricted” mode to hide it. Then you can generate download keys for those buyers that have purchased in the last 30 days.
Thanks