Can a person chose to believe something other than what they already believe?

Not as such. But you could chose to distrust you own judgement on a matter of belief and to trust a person or institution which has a better claim to knowledge and authority than you do.

To give a secular example, if you feel fine and believe yourself to be in good health, but your doctor tells you your arteries are clogging up and you need to change your diet, you might decide to follow your doctor's advice on trust even though your own belief contradicts him. In time, if your doctor's advice proves beneficial, your beliefs might come into line with his. 

Sure. People often leave one religion for another, and hundreds of atheists find God and become Christians every year.

No, we cannot just decide to believe something.  Beliefs are based on some kind of evidence or experience.

In a way, yes.  It is a funny reality of human psychology that we can change our mood and beliefs by 'pretending.'  If you do an experiment where you ask people to pretend to be happy they will tend to feel happier.  If you ask people to pretend they are enjoying an activity they will tend to enjoying that activity more.  If you ask people to defend a position they don't believe in they will tend to start believing in that position.  There is good evidence that our beliefs follow our behavior as much as our behavior follows our beliefs.  Case in point, Ryan Bell, a very devout minister, "lived as an atheist for a year," he is now an avowed atheist.

Who is choosing?

To change belief implies a preceding change in the self that mutates the past. 

To be open to observing one's own assumptions socratically: one would need motive, maybe when one's beliefs cause one to fail when one desires to succeed. 

It probably can happen in different ways.

When missionaries have gone to primitive areas, the natives might have been impressed with the technology that the missionaries had.

Or, sometimes people can just be impressed with the zeal of the proselytizers.

Sometimes, it can be a sort of self-hypnosis.  Mormon missionaries say that their spiel wasn't working on me, and they asked me to just hold the Book of Mormon for 10 seconds.  So, I humored them and, as I held it, I slowly counted to 10.  I suppose it must work on *some* people or they wouldn't keep doing it. 

believe it or not, belief can change organically or perpetually through manipulation, information, experience, and exposer to influence. some people possess enough self awareness to realize their current belief is irrational, or immature. many times people grow out of their juvenile understanding and childhood belief changes naturally as they grow. 

choose do u see the spelling?

they CAN BUT ONCE PEOPLE ARE SAVED THEY WONT GIVE THAT UP...ETERNAL LIFE WILL BE AWESOME!!

All you need is an open mind and the recognition that you do not infact know it all, that there is always more to learn

You need to be thoroughly convinced that what you believe is wrong. For many people, that takes a traumatic event of some sort. For many lions, it takes a 2x4 upside the head.