I wish I were so optimistic about that, but I have seen enough examples of experienced and gifted Buddhists "falling down" to think it not so simple.
Some changes happen just through sitting ... to wit, our sense of "self" in conflict with "other" softens and sometimes fades fully away. Real, abiding changes occur to us "automatically" through this Practice.
But as far as keeping to the Precepts ... avoiding excess attachments, lust, anger, gluttony etc etc ... the Zazen sure helps and, sometimes, may be all that is needed. But at other times when temptations are in front of us, much work and willpower may be required too ... though facilitated by Zazen. I have seen few (really, I doubt any) living breathing cases of someone who "got enlightened and was thus 100% free of greed, anger, etc. and that was the end of it" outside of Buddhist story books and their hagiographies of long dead Buddhists.
The subject of "hagiography" came up on another thread today.
Ah, sure, maybe we will all be "Perfect Golden Buddhas" one day, some lives down the road ... but in the meantime, we are just Bodhisattva Bozos on the Bus, doing what we can each day.
That does not mean this Practice is not worth it, however. It is, and is radically life changing. It is just that one must stay "on one's toes" a bit. I sometimes compare Dogen's view of ongoing "practice-enlightenment" more to an ongoing dance or "bicycle ride" than a fixed "once and for all you are done" state. Life is an ongoing dance and, no matter how gifted the ballerina, there is always the chance of a stumble at the next turn ... with some days smooth and some days more a struggle. Same with riding a bike ...
But don't give up dancing or bike riding, my friend, just because ya might take a small spill now and then! (at least, of course, until we are all perfect Golden Buddha Ballerinas and Bike Riders).
(By the way, some folks get real real mad at me when I say they should drop the dream of "perfect Buddhas and Ancestors" and "once and for all enlightenment". However, that is the only way to be at one with this perfectly what it is life ... and to realize that we have the choice of "Buddha" or "delusion" in each new moment, with each new step. It is up to us with each new day.)