I got a bit closer, but couldn’t actually resolve the issue: .cpr files are associated with Cubase.Project entries in the registry. I found two entries referring to Cubase 9.5
Both have command.default = “C:\Program Files\Steinberg\Cubase 9.5\Cubase9.5.exe” “%1”
It seems as if it should really be command.default=“C:\Program Files\Steinberg\Cubase 10\Cubase10.exe” “%1”
However, the associated command.command field contains an “encrypted” field that appears to also be in use.
Changing the command.default entry to point to Cubase 10 enables Cubase10 as the default association, but loads Cubase 9.5 without any prompting.
Bottom line: something prevented Cubase 10 installed from updating the default project open commands for file associations.