You are right, this is beyond ability of most users. It just could be done by Graphisoft, that the version-number is written in the Icon.

You can easily fix that. Just go to the folder /Applications/GRAPHISOFT/Archicad 28 - Technology Preview/Archicad 28.app/Contents/Resources and open Archicad.icns and add a small 28. Go to ArchiCAD App-information and change the icon to the new one. ...