It doesn't really change. It happens because of the accent, probably, most of the people in Brazil change the sound "o" to "u" sometimes, but not always. For example, in the south of Brazil, it almost does not happen. But in the southeast and in the center of Brazil it usualy happens. I know it may be confusing, but it's how we say it. When you familiarize with this you wont notice the difference.

Sorry, I didn't understand your question.

To reiterate what Lucas said, it does depend on the region but the final unstressed "o" does turn into a slight "u" sound, depending on the person, area, etc. But at the same time I've noticed w/ these same people who say "u", when emphasizing or speaking slowly, they will say it as an "o".