Set the check state of the task to match the current startup state (if upgrading)

Question

My software installer allows the user to start the application when the user logs in. It does that like this:

Root: HKCU; Subkey: "Software\Microsoft\Windows\CurrentVersion\Run"; \
    ValueType: string; ValueName: "appname"; ValueData: "{app}\appname.exe"; \
    Tasks: startup

It is based on the answer here (https://stackoverflow.com/a/40147393/2287576). But the software itself allows the user to modify the start-up state (by programmatically adding/removing the same registry data).

How can I adjust the Inno Setup Script to detect if:

1. It is upgrading and
2. If it is already in the Startup list

I would like the check state of the task to match the current startup state (if upgrading)