Hot Lips change

I think this was to show her character change over time. We can assume Duke and Hawkeye get there fairly early in the war and leave when it's almost over. That could be enough time for Hot Lips to have a change of attitude.

You're absolutely RIGHT ~ I thought the SAME thing!
And I also thought {and noted elsewhere on this message board} that the whole 'football game part' of the film was not only out of the film's "character", but not funny and a complete waste of time.
'You can't HANDLE the truth!'

I think she just finally accepted the fact that she couldn't change the 4077th, so she joined in with the shenanigans.

The more I think about this, the more I think the change is actually in line with her character, at least in one respect: she starts out as a "cheerleader" for the Army, rigidly enforcing protocol, which irritates the anti-authority characters. But after she realizes that Hawkeye and Trapper are doing whatever they want without repercussions, it must seem to her like they are the ones running the show. So Hot Lips decides to become their "cheerleader" (figuratively and literally) since she's a follower of whomever is in control. The whole thing makes Army authority seem like even more of a joke and shows that the subversive characters have taken over.

Wow, I'm really surprised no one gets this character change. It all seemed very clear to me when I first saw it that without the influence of someone like Major Burns, and with increasing familiarity with the "cool" people, she would free herself up to be what she always secretly wanted to be: one of the crowd. She's like the Catholic school girl who gets out in the world and discovers that there's fun to be had. With no one else around her to reinforce her old uptight ways, she found that she just wanted to have fun like everyone else. There were even fore-shadowing events in the film that demonstrated this change. For instance, when she is praised by Hawkeye for being a pain-in-the-ass but a "damn fine nurse", you could see her delight at the compliment. Then there were scenes of her hanging out with the boys during their poker games (I admit it's been a while since I saw the film), so she was already starting to chill out a bit. By the time of the football game, she was fully on board with the gang. There was nothing unusual about this at all. Remember, also, that the film was a thinly disguised commentary on the Vietnam war, and there were a great many people in the '60s who were gung-ho for the war only to realize that it was wrong and eventually joined the majority of people who opposed it. The Hotlips character perfectly symbolizes that type of person.

Aspects of her treatment were misogynistic (the shower business especially) and it would have been more convincing if she resigned or at least demanded a transfer to another unit. I too find the character change unconvincing.

"Chicken soup - with a *beep* straw."

looking at it now, yes she was abused horrifically.

and her transition... I think she let go trying to be in charge and just learned to have fun. She allowed herself to be stupid.