I don't want to be the oxymoron of the group but...... if everything is going to be about just having fun, where is the discipline going to come in for the guys to push themselves, get in shape, learn and be prepared?
First of all, Coach Heupel isn't going to recruit guys who aren't already 90% motivated from within. I think the answer to the question beneath your question is that "fun" came from brain science discoveries, not changes in culture or values.
All discipline has to be rooted in motivation. That motivation might be fear of consequences or displeasing someone important to you. It could be irritation ("I'll do it--but just so you'll shut up!"). Those are all example of being motivated by negatives to avoid something even more negative.
Motivation for discipline can also be positive, like to help you achieve personal goals, or achieve team goals.
Fatigue tends to break down all discipline, because when we're under physical or psychological stress, our brain system defaults to survival mode--conserve energy and think only about getting through the next moment. That's where the "having fun" tactic pays dividends: if you're
enjoying your physical stress, your brain doesn't start throwing those survival switches--each of which will
decrease your ability to push yourself physically, engage mentally, learn, or focus on finesse things like technique.
The physical sensations of falling off a cliff and riding an amusement park ride are approximately the same. But how your brain responds to them makes a huge difference. One is, "If I survive this, I'll avoid heights for the rest of my life!" But if you're having fun,
"This is awesome! How long is the line to ride it again?"
So it's not really a cultural change; it's a tactic, based on discoveries in brain science, to maximize the time you get with players in practice and maximize their performance during game situations.
But players (like golfers) still have to experience (in order to learn from) those pressure moments when you can't avoid knowing that "if I screw this up, we lose." But even then, some players instinctively "run that unique stress through their fun circuit" and actually
relish those pressure moments--maybe for their own adrenaline high. We can all name some VFLs who had that gift!