Originally Posted by channa

This is not correct. They prefer rebooking on Star Alliance carriers because it's less expensive for them, but they can and should rebook on other carrier (e.g., DL, AA, etc.) in the event that it's significantly better.

Per the Star Alliance Rebooking Rules they are to book on OAL carriers if it is in the customer's best interest:

Rebook in the following carrier order, depending on frequency of flights and the customer’s best interest:

* Own airline
* Star Alliance member airline
* Non-Star Alliance member airline (OAL)

Since the DL flights were significantly better time-wise than the Star Alliance flights offered, and the customer requested them, UA should have booked them on DL.

We don't see UA do this too often (it's usually a game that CO likes to play). I would write into CS and point out that not only were you inconvenienced, but UA failed to follow Star Alliance rules to mitigate that inconvenience.