flightaware.com/live/flight/UAL259
I’ve always been interesting by airlines using B744s domestically within the USA.
Looks like a substitution.
Hint: If you are talking about a specific flight, click on the flight (assuming it has already departed) and the click on the URL that appears. That way, for time eternal, people will be able to click on the link and get the specific flight you are talking about.
flightaware.com/live/flight/UAL2 … /KORD/KDEN
If you look at the history since Jan 1 (when UA seems to have changed the rotation), that flight has been a 747, 757,767 and 777 at various times. Anything that’s lying around ORD at the time 
When there’s disruptions it is more likely to get upgraded, that way they can get people who have had their flights canelled or missed connections to Denver and then reach their destination from there.
You make it sound like they care!