What if I change my airport for getting Vietnam visa on arrival?

There will be no problem at all.

Vietnam visa approval letter works at three international airports of Vietnam (Noi Bai in Hanoi, Danang in Danang City and Tan Son Nhat in Ho Chi Minh City). So, after having your visa approval letter in hand, you can enter Vietnam via any of such three airports to get visa stamped with that letter no matter which airport you put in your Vietnam visa application form.