Why don't you want to upgrade the app to v10? They already own a v10 license, right? You buy the license, the media is "free"...
The conversion should be absolutely painless. You open the app with v10 WindowMaker. It will complain that it was developed with an older version and ask if you want to convert. If you answer yes, it will ask if you have it backed up. Then just set back and let it do it... It truly is that simple. 5 minutes, max.