Thanks @paullimapa ;
No screen shots sorry as this is for one of my users and she has finished for the day. I have approximated what I'm seeing using my PC (where it works correctly).
Yes I have tried all of these.
* Associate using the specific version acad.exe file, Change the default app by file type, Associate to AutoCAD Launcher from File Properties:
For all of these methods, after I browse to the executable for AutoCAD 2024, Windows jumps back to the (uninstalled) old version. I even tried to trick Windows by selecting a completely different program (Notepad) but it still jumped back. What I see is like in the screen shot, where the app in the arrow says 'AutoCAD 2022' with no icon, and the 'Always' button is missing, and AutoCAD is not listed as an option in the list below.

* Associate DWG files with the AutoCAD DWG Launcher:
I tried this and it made no difference.
* Use "Send to"-method:
Not really a solution in this case as I want to set the Windows file open default so that it then also works in Vault.
So this may be more of a Windows problem than an AutoCAD problem, but I posted here in the hopes someone had already solved this problem.