Heya,
This idea has been on my mind for a while, so I thought I'd go ahead and see how it flies as a thread topic (and forgive me if this issue has been dealt with elsewhere).
It's interesting to me to watch how the characters of Ennis and Jack evolve in the film in relation to the identity of being a cowboy. And, their relationship to an identity as a cowboy seems to reflect each of their characters in broader terms. Both Ennis and Jack start out as "real" cowboys... both in terms of look (clothes, hats, mannerisms, etc.) and work (really working as ranch hands, really participating in rodeos, etc.), but this changes for Jack. Clearly, once he marries Lureen his identity in terms of work shifts. Selling farm equipment seems to be one step beyond "true" cowboy work... too much time indoors, a little too suburban, etc. And, this is early in the story too... even by the reunion Jack has already begun to slip away from his earlier identity. Maybe this is why we never see him wearing his original black hat (with the twisted leather band) starting with the reunion scene. His cowboy gear after this point seems more like a fashion statement than anything else. His big black hat during the final argument scene seems like an exaggeration or even a caricature of his original hat. Ennis, not surprisingly, is very different in his relation to being a cowboy since he never stops working as a ranch hand. But, he stops wearing classic cowboy hats (or wears them less and less) towards the end... in favor of that less-than-flattering hat that looks something like a fishing hat. But, his identity as a cowboy in terms of work remains solid. So here, not surprisingly, we have a situation where Jack's character is more changeable than Ennis.
But, I wonder what this issue in the film (I feel like this is less pronounced in the story since some of this is based largely on visual perception) is saying about western/ cowboy life and culture through the decades that the film covers.
And, clearly Ennis and Jack aren't the only cowboys in the film... it's interesting to see the range of identities that seem to fit this term as depicted in the film.