You’re correct - the 2024-native version is a new, separate product. The 2024 compatible version is the same 2020 version that’s been out for a while, which has been patched for 2024 compatibility. So if you already own the A300-600, you don’t have to pay anything to get 2024 compatibility - it’s a free update to the product you have. You only need to pay if you want the “premium” version that is fully 2024-native.
Edit - and if you are purchasing it now, you would want to go for the “old” 2020/2024 version if you intend to keep using 2020 and want to have the plane in that version - the 2024 one obviously won’t work in 2020…