Fiqh-us-Sunnah Volume 3, Zakaat and Fasting, Fiqh 3.139C.

Section : If one has the intention, while he is fasting, to break the fast, he in effect voids the fast even if he does not actually eat anything.

This is because the intention is one of the pillars of the fast and, if one changes his intention, he has nullified his fast.