OK. I tried out a game I made in DBPro Upgrade 1.075 on a Windows 98SE computer (with DirectX 9.0c), and a dialog popped up:
This program expects a newer version of Windows.
Upgrade your Windows version.
Then I installed DBPro 1.054 (which was in my order) on another computer (which runs Windows Vista), changed the code a little, and re-compiled it. That version didn't work on the 98SE computer either, and the "Upgrade" dialog was replaced by an "Error" dialog.
It would be nice to know which operating systems are compatible with my games (there are still a lot of Windows 98 users).