Did you purchase the flight from United prior to the 3/3 system conversion? Did the fare rules of the ticket you purchased allow changes with a fee? Did the fare rules of the ticket you originally purchased require a Saturday-night stay?
If what's actually happening under the hood is that your ticket is being exchanged for a brand-new ticket using whatever the current fares are in the market (including whatever rules they include) then what you describe makes total sense, even if it seems a little unfortunate.
Yes, Saturday-night stays and extremely high hub-to-hub fares were unusual on sUA, common in sCO, and are a common practice in the new UA.
That's not to say that sUA offered perfectly flexible fares just prior to systems integration, though. They did away with the $55.81 stopover in DEN on most fares a year or two back. And it was back in 2010 that UA started applying extremely tight routing rules,
http://www.flyertalk.com/forum/unite...strictive.html.