Why do movies change music or scenes after theatrical release?

I was wondering if anyone knew why movies change things in between theatrical release and release to dvd?

For example: I went and saw the movie Center Stage the first day it was out (YEARS ago) and remember this slightly touching conversation on a ferry. I bought the dvd and that entire conversation had been cut out. it was weird.

Or when I saw the movie House Bunny. I saw it after a few weeks it had been out in theaters and the scene where the girls are getting a make over, the song "When I Grow Up" by the Pussycat Dolls was playing (I remember because I can't stand that song). Well I just watched it on cable and some Avril Lavigne song played and there was zero Pussycat Dolls.

Basically my question is: why is it that these things are changed? I mean, obviously the studios already made money--why change them?

Thanks!