I personally know of dozens of these cases where growing up in the 80's and 90's these people were in fact "byu fans" but are now "Ute fans".
It's not all that complicated, people like to feel a part of the winner. It's fun, it's exciting and it's where the majority of the sheep tend to flock.
How is this any different from the late 70's, 80's, through mid 90's where the lion share of the state of Utah were "byu fans" in the sense they wore Y gear, went to an occassional game and tuned into most games on TV.
I went to plenty Utah games as a child and remember just how sparce Rice stadium was despite its 32k ish capacity. I remember being with groups of people growing up and there being the lone Ute fan.
I now see the opposite. My work offices have been overwhelmingly Ute fans and most young couples/families in my wards in recent years are "Ute fans".
Most humans are followers and wanna run with the pack, very few understand loyalty and have the character to stick with the losing program they were once "fans" of.
I'm stubbornly loyal and prideful. So if for no other reason but pride I will never join my friends and family who have swapped loyalties to the U.