127:10 If a person vowed to fast one day or multiple days and he did not specify which day or days, even if he accepted it upon himself at mincha to fast the next day, if some compelling need required him to eat – such as if he was called to participate in a meal celebrating a mitzvah even though he is not connected with those celebrating, or if an important person invited him to eat and it’s difficult to refuse the invitation, or if he is distressed – then this overrides his commitment to fast and he may eat on that day, even if he already started to fast. (He need not have his vow absolved – Mishnah Brurah 568:10 – though he must fast at least a little that day – MB 568:9.) He must then fast on another day in its place. Such a switch is permitted if when he obligated himself in the fast, it was just to fulfill his vow; if he did not vow in the first place and only accepted upon himself at mincha to fast the next day, then even if he regrets it later, he may not move his fast to another day.
127:11 If one made a vow to fast on certain days and he also accepted the fast upon himself at mincha, he may not move the fast to another day.