@donho FYI, I just installed Notepad++ 7.3.2 64-bit version where there had been Notepad++ 7.3.2 32-bit version and it warned me about removing the 32-bit version while noting it would preserve settings.
I have done this several times since my original post - thanks for the changes to accommodate this.
However in this case I then ended up using the View Source command in Internet Explorer and got this error:
C:\Program Files (x86)\Notepad++\nppIExplorerShell.exe
Windows cannot find ‘C:\Program Files (x86)\Notepad++\nppIExplorerShell.exe’. Make sure you typed the name correctly, and then try again.
Turns out the 32-bit installer had set this value at some point in the past in the registry and it was not modified or removed when I ran the 64-bit installer:
HKEY_LOCAL_MACHINE\SOFTWARE\Wow6432Node\Microsoft\Internet Explorer\View Source Editor\Editor Name
(Default) = C:\Program Files (x86)\Notepad++\nppIExplorerShell.exe
Since the 32-bit program is removed, there is no C:\Program Files (x86)\Notepad++\nppIExplorerShell.exe
Further there is no 64-bit version of nppIExplorerShell.exe in C:\Program Files\Notepad++
You can, of course, replace the default value with “C:\Program Files\Notepad++\notepad++.exe” and IE will launch npp as the viewer although it will likely throw a security warning.