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After installing 1.2103 in a 64 bit OS, I wanted to go back to 1.2101 for testing.
After installing 1.2101, UCCNC will not run.
I'm guessing something gets changed when installing the 64 bit version that doesn't let you go back to the 32 bit versions?

Gerry, it is because the app config file is there in the UCCNC folder.
It is the UCCNC.exe.config file which is required to be there now to let older plugins to run which plugins were compiled with .NET 2.0 and have code in different app domains,
and so the .NET 4.0 security policy does not allow those plugin .dlls to be loaded in by the .NET 4.0 UCCNC.exe without having a flag set in the app config file.
So, the UCCNC.exe.config file is installed there to tell the .NET 4.0 framework to allow these dlls to load.
But the app config file is also telling the .NET framework that the UCCNC application is using the 4.0 framework and when you installing the 1.2101 version back,
that installer does nothing with the app config file, because the 1.2101 release did not need this file, because that file was not needed.
So, the 1.2101 installer does not delete the app config file and that release relies on the .NET framework 2.0,
so the app config file is still there and when you try to run that UCCNC.exe then the app config file tells Windows to use the 4.0 version,
while the app was compiled with the 2.0 framework and so the app can't run with the 4.0 framework and so is why the immediate crash.

The simple solution is to manually delete the file UCCNC.exe.config from the UCCNC folder if you reinstalling an older than 1.2102 version and then the older version will run...

One habit I've gotten into is doing a complete copy of the UCCNC folder before I install an update. If I want to roll back to the copy.

Or uninstall first, because the uninstallation will remove the app config file.