Edit:  After doing some research, some hints from commenters and a significant bit of trial and error I now have. MSFS back running. It seems not to be caused by one error but instead was caused by a series of cascading errors, which began with the HW upgrade. 
 

Thanks in advance for any help given or offered. Windows 10 120 Gigabyte MSFS 2020 installed to G:\MSFS2020. 
 

Today I upgraded both my Mobo and CPU. Windows came up and seemed happy. I went to run Flight sim and it balked then said it needed to “update”. Ok I think   go ahead. 

After 3 min or so an error message piped up and said it could not install the helper app to C:\program files\WindowsApps….  Wait. My install is on G but it’s trying to set up a new instance on C. Now I’ve uninstalled and tried to reinstall. The Store sees the 120 gig install on G:\MSFS2020 and says that is where it will install Flight Simulator. The installer whirs and spins for a couple of minutes and reports 1.2 Gigs installed to G:\MSFS2020, Go ahead and play now. Click play, pause 30 seconds and then the error message about C:\Program Files…. Once again it’s trying to install the launch stub to C:\ NOT TO G:\ and I’m stuck and don’t know what to do next. 
 

HELP PLEASE😂😂😂

Edited May 15, 20224 yr by btacon

This might help you. I had to do something similar when I got a new drive a couple years ago for Forza Motorsport. I buy everything that I can on Steam now. The Microsoft Store handles installing things so poorly in comparison IMO. Good luck!

https://answers.microsoft.com/en-us/windows/forum/all/not-able-to-uninstall-old-microsoft-store-games/4557b56f-4d17-4c12-b30d-5cae1eb88fcb