Change site URL should alter the table or create .htaccess?

I don’t know where else to post serverfault, superuser or here, but Please accept my apology.

I got a very big tast I would say because I never done this before myself.

There is a site which they are running on WordPress for over a year and they just bought a new domain name.

So they asked me they want to just change the domain name of the http://oldsite.com to the http://newsite.com what happen to myself before when I have the same situation I just move from one server to another server and changed the domain name at the same time with wordpress they store the URL of the images like absolute url not like relative so sure that many images will not be display.

So in his case that they are still going to use the same server which means all the picture will still be in the same folder /wp-content/uploads/*

So my question is how can I change his domain name without affecting the pictures in his site after changed the domain?

Doing alter table? or .htaccess mod rewrite?

if there is any better option please suggest me I would be so much apprecaited 🙂

Thanks and again please accept my apology for not knowing where to post.

UPDATE

The only and most important thing is that how to make all the pictures from all the posts still appear after change the domain.