How much are you willing to change for someone you love?

I used to believe that if someone truly loves me, he will accept me for what I am and wont expect me to change so he could be happier. This is how I was when he met me, anyway, so why would he expect me to be different once we're together?

However, I've been observing "successful" partnerships for some time, and I noticed that either one or both of them have changed since they've been together. 

One friend who used to be a successful career woman and was working as a department head for an IT company before she got married has now quit her job and is quite content with being a plain housewife - she even learned how to cook and do laundry 

A relative who used to be a devout Born Again Christian when she was still single has now converted into another religion (I suspect it's a cult, actually  ) because of her husband who also just recently converted.

A college friend who used to be so outspoken and always stood up for her rights before is now so tame when dealing with her husband. Common friends tell me that she doesnt even answer back and just cries silently when they fight and the guy shouts at her 

And so it has made me wonder - for a relationship to be successful, does one of you have to change to adjust to the other person? What if the change you make in yourself makes you miserable, is your relationship still considered successful as long as you stay together?