Upgrade mode will prevent the installer .... (pgAgent)

I can't interpret this message, or figure out whether I should select the option or not.
My main problem is that my intuition is that checking "upgrade mode" will perform an upgrade, but the text of the message seems to say that checking upgrade mode will NOT perform an upgrade.

Since I don't know if images are OK, I'll try to transcribe the message:
"Upgrade mode will prevent the installer from modifying the database cluster by loading pgAgent SQL scripts in preparation for upgrading from earlier releases of PostgreSQL using the pg_upgrade tool."

Context: Running the Windows Installer for v12 with v11 still installed on the machine. After the main install I selected several packages, including pgagent, which I also selected for v11. I have a mild preference for treating 12 as a clean install without migrating anything from 11, since it was just a test database.

I shut down postgres 11 and its pgagent from the Windows services dialogue, and then changed them to manual start. My plan is to uninstall them if things go OK with 12. Both are currently configured to use the same port.

Thanks.
Ross