MCT does not necessarily equate to SCT (Sensible Connect Time). MCT is simply a carrier's willingness to accept the risk of a reroute, e.g., rebooking you on the next available service and, depending on the circumstances, paying for a hotel & meals.

MCT's are written somewhere. It is simply that agents don't need to know that anymore. They are trained to rely on the GDS. If a connection is bookable, it is within MCT. In OP's case, the connection is bookable and even with the change, it is still bookable.

Whether flights at or just above the MCT are for you is a matter of your schedule and your risk tolerance. Short MCT's are great for people with a very tight schedule on either end of a ticket. Not so good for people with some more time to burn, who may have mobility issues (including little kids).

Most carriers, including UA, will automatically book you onto the first service at or above MCT. But, domestic connections can be as long as 4 hours. The problem with leaving it to a late arrival is that there may not be seats on the next couple of flights and, more likely, the seats will be the worst on the aircraft.

It really pays to watch this and, when there are schedule changes, to make certain that you are comfortable with what is proposed.