I don't know what the minimum connecting time is for Denver's airport, but that sounds incredibly tight for any airport. Your second flight will usually start boarding 30 minutes before departure, which means that they will probably be boarding by the time you get off the first plane, assuming it is on time.
If the flight time has changed significantly, you may be entitled to a change of flight at no cost. Not sure since this is through Expedia, but I would definitely call them and see what can be done.
One of the pluses of flying any airlines except Southwest is that you have assigned seating. As such, you don't need to be at the gate right when boarding begins. The biggest practical issue with a tight connection of this nature is that overhead bin space may be non-existent.
You are correct that some changes allow for no-cost changes. Generally, that includes: 1) flight times that are changed by over an hour, 2) connections that are shorter than the minimum connection time, or 3) carrier changes such as being move from a legacy flight (United) to a regional flight (United Express). None of those cases would apply here.
Where the ticket was bought does not affect the ability to change flights for free when an applicable situation happens. But, if you purchase from a
travel agency (regular or online like Expedia), you do have to contact them to request a change. This adds more people to the mix, which can be frustrating. That's the reason why it's almost always best to purchase flights directly from the airline. The only reason to book with Expedia would be if it saves you money or if their rewards program offers some tangible benefits.
According to this thread in FlyerTalk (
https://www.flyertalk.com/forum/uni...e-logistics-domestic-international-den-9.html) it is 30 minutes for domestic-to-domestic.
I do think
SirDuff is correct in stating that 30 minutes is the minimum connection time in Denver. You have two things working in your favor.
First, your United Express flight is on a smaller regional aircraft. It looks like a CRJ-700. It only has 70 seats (18 rows with 2x2 seating in economy class). Even if your at the back of the plane, it won't take long to deplane when compared to a "normal" single-aisle plane that can have 160-200 passengers.
Second, your flight isn't until the end of September. That's an eternity when it comes to airline schedules. It's possible that your flight times could be changed multiple times between now and then. I recall purchasing a ticket about five months ahead of the trip and having 4 different schedule changes.
I've been working in and following the travel industry for several decades. I totally understand the concern with these tight connections. Just remember that if you misconnect, it's up to the airline to fix the problem. As such, they won't purposely sell tickets for itineraries that result in routine misconnections. If United is selling a 38 minute connection, it means they believe it is possible.