flight works for you then you book in main and get the UG to first about 5 minutes after booking. (the note about which certs work is in the lower left of my posted image)

If there is no language mentioning upgrade certs on any given itinerary, then if you use a regional it impacts your place on the upgrade list (it doesn't guarantee anything).

As a diamond with a regional certificate you'd place high on the upgrade list.
Here's basically the hierarchy for you if you're on the upgrade list.

Diamonds with regional certs who bought economy comfort
Diamonds with regional certs who bought a ticket in main
The tiebreaker would be million mile status.

So for me, if I'm using a regional and I'm not guaranteed an instant upgrade, I pay for economy comfort because it puts me highest on the list.