Sorry, but fast paced offensive teams are not built to be great defensive teams. Just not how it works. So if you want BYU to become a great team on D, they need to switch tier style of play to be more similar to Utah.
SO I am not saying BYU can't try to improve D while staying in the same O format, just saying don;t expect more than minimal changes unless they revise that O.
Great defensive teams start with tempo. Think Wisconsin. They basically take the air out of hte ball, make you play D was 30+ seconds, then get back on D quickly after a shot. No one wants to see BYU do that.
BYU just needs to get a little better in the post, and more efficient on O, and the D will sort itself out more. Better shots, give up fewer fast break points. Small changes.