"Become"? I didn't observe any difference to what we saw in TOS, or ENT, or what we see today...
And I totally buy the backstory of why Worf became a serious guy. Being a serious guy is so completely anti-Klingon on all the other occasions where we have a chance to observe Klingons... The first officer on the Klingon ship that Riker temped on in "A Matter of Honor" was basically the only other "work is all I live for" Klingon out there, and he, too, had an excuse for it: he wanted, yet at the same time didn't want, to distance himself from his loser of a father. His shipmates were the antithesis of Worf, though.
I mean, normally a person who acts like Worf would be doing so in order to play the tough guy; "See, I'm all rigid military manners because I'm a badass soldier!". But Klingons do that very thing the exact opposite way. And Worf didn't want to distance himself from his heritage, quite to the contrary. So an alternate explanation was very welcome.
Of course, the kitchen psychology that Worf applied on himself isn't likely to be the whole truth. But it's still an important formative element of what he is, by definition.
...Also, a moment of self-analysis in the presence of you special other is a classic formative element in itself. It need not have to be the truth, or even have the kernel of truth there; but rationalizing yourself to another person is always interesting and significant, and may lead to great things.
Timo Saloniemi