Would my prayer be accepted if I swore by Allah to recite only "Qul Huwa Allahu Ahad" after Al-Fatiha, and then changed it? And do I owe expiation?

The obligation to recite Surat Al-Ikhlas in every rak'ah after Al-Fatihah is contrary to the Sunnah. It is preferable not to adhere to it and to follow the practice of the Prophet, peace and blessings be upon him. Therefore, breaking this oath is preferable and good, and the prayer will be accepted, God willing. However, an expiation for the oath is required: feeding ten poor persons, or clothing them, or emancipating a slave. If you are unable to do so, then fast for three consecutive days.