It sure is a measure of the power of the movie that when we aren't busy agonizing over the actual tragic ending, we're agonizing over the equally tragic alternate ending that might have occurred if the actual tragic ending hadn't.
Whenever an ambiguous Brokeback situation presents more than one possible plausible interpretation, my longtime policy has always been to always pick the least depressing one. I figure there's enough undeniable sadness here already, so in the case of uncertainty I can treat myself to a happy view as long as it's a reasonably believable fit in the story.
But however you interpret it, there's no good way out of this.
1) Jack decides to quit unless Ennis changes, Ennis can't change, so they would have been quitsville in November, but then Jack dies.
2) Jack decides to quit unless Ennis changes, meanwhile Ennis resolves to change, so they would have been happy in November, but then Jack dies.
3) Jack resigns himself to sticking it out, Ennis has no intention of changing, so from November on they would have stayed status quo indefinitely, but then Jack dies.
4) Jack decides to quit unless Ennis changes, meanwhile Ennis resolves to change, then Jack dies. But wait! Turns out Jack faked his own death to teach Ennis a lesson and escape Lureen, he shows up in November, and they live happily ever after!!
Ordinarily, my policy would lead me to choose 4, but unfortunately there's that darn believability clause. So that one's out. No. 3 is plausible but not very exciting and ends just as sadly, so guess I'll eliminate that one, too. That leaves 1 and 2. Which is least depressing? It's a tough call. But in 2 at least the alternate-alternate ending (the parallel universe one where Jack doesn't die) would have been happy, unlike 1. And even with Jack dying, 2 leaves a tiny shred of hope for Ennis. It's even sadder in the short run, full of classic tragic bitter irony, but at least it ensures he has learned something about the importance of love over homophobia. So maybe just maybe he could be happier in the long run. So I pick 2.
He *has* changed after the lakeside scene, to me the Cassie scene shows that very clearly. He seems to have reached a point where pretending to be straight just isn't worth any sort of effort any more, but he doesn't look like a guy with a purpose....... in both the Cassie scene and at the beginning of the scene where he collects the mail he looks like someone defeated and cowed by life, almost.
I agree that dumping Cassie scene shows he's stopped pretending to be straight. Still, he's really glum. He told Jack at the lake he can't stand it no more, which according to his axiom means he has to look at fixing it. But how? Then Cassie tells him, "Girls don't fall in love with fun." Suddenly, he quits eating pie, looks up with an expression of realization. He more or less ignores Cassie and Carl's departure and gazes off into the distance, pondering. What?
I believe that line triggers some kind of epiphany. I'm not sure exactly what it is. I think I have seen Ruthlessly present a view of this, don't know that I fully agree with it, but I'd like to see it again. In any case, that's the clue that Ennis has decided to change. And unlike Mikaela, my impression is that he comes out of the post office looking all perky (well, faintly perky). It's hard to tell, it could go either way, but (my policy! and the logic that suggests he figured out something good in his epiphany) I choose upbeat.
So two seconds of upbeat, anyway, 'til he sees the postcard.