There were two plans. Utah was plan b. They were in if the Big 12 raid (plan a) failed. The Big 12 raid was contingent on texas joining, otherwise it was a no go. CU was in all along and wanted out badly (just like Nebraska). In fact they preferred that Texas not join because they hated texas. Their fans celebrated like the wall had come down when they got invited.
Those are the facts as were reported by numerous members of the media at the time.