Have there been changes to UA SDC procedures recently?

I’m certain that I’ve changed to earlier or later flights the same day, same origin and destination, sometimes different connecting cities, occasionally the night before ticketed morning flights, without additional charge, including when the change is to a different booking class, MANY times. I recall reading that within 24 hours (of flight times) all booking classes “even out” or “zero out”.

Trying to do a SDC for tomorrow’s SLC-IAH-LGA itinerary to one of two SLC-DEN-LGA itineraries because SLC-DEN-LGA is 2 hours less flight time than SLC-IAH-DEN. One DEN connection would allow me to depart at the same time and arrive 2 hours earlier, the other would allow me to depart 2 hours later and arrive at the same time. LT Gold, both SLC-DEN-LGA itineraries are showing plenty of available seats.

Full desktop website errored out twice. Mobile app showed significant additional fares. Just spoke to a UA phone agent and her supervisor, both told me that since there is no “T” class availability in the flights I’d like to change to I would have to pay $196 or $133 to make the change.

I’m pretty old, Medicare eligible for 2 years, really hope I’m not remembering this incorrectly. Thank you.