changeing the site title off will affect SEO or it will improve the SEO? added new keyword will affect SEO?
Changing the <title> element will likely always affect your rankings... it might help, it might hurt. Depends on the change.
True Your site title is a first point of call and very important for each and every page - which all should be different btw.
I change my titles all the time. Lately I notice Google liking longer titles instead of shorter ones. I also find it beneficial that your title tags should reflect your site content, with header tags throughout the page that reiterate the keywords utilized in the title. Then you would also expect to have the same keywords from the title in the content as well (lookup keyword density for more info).