Is it permissible to change a vowed action to something better?
Yes, it is permissible to change the action mentioned in a vow to something that is better.
Shaykh al-Islam Ibn Taymiyah said in al-Ikhtiyaaraat (p. 329):
Whoever vows to observe a specific fast can change it to a better time. End quote.
For example, if a person vowed to fast on Tuesday, then he may fast on Monday instead because that is better.
The Standing Committee was asked about changing a vowed action to something that is better than the first. They replied:
The basic principle is that if a man mentions a specific place in his vow, such as a vow to give charity or build a mosque in a specific place, he is obliged to fulfil that vow in the place and manner specified, so long as there is no shar’i reason not to do so, unless he changes it to a place that is better, such as the two Holy Sanctuaries (Makkah and Madeenah). There is nothing wrong with that. This is indicated by the words of the Prophet (peace and blessings of Allaah be upon him) to the man who had vowed to sacrifice a camel in Bawaanah. He said: “Were there any idols of the Jaahiliyyah there that were worshipped?” He said: No. He said: “Were any of their festivals held there?” He said: No. He said: “Then fulfil your vow.”
He commanded him to fulfil his vow in the place that he had specified because there were no reasons why he should not do so. The fact that a vowed action may be moved to a better place is indicated by the fact that the Prophet (peace and blessings of Allaah be upon him) said to a man who had vowed to pray in Bayt al-maqdis (Jerusalem): “Pray here.” The man repeated his statement and he said, “Pray here.” The man repeated his statement again and he said: “It’s up to you.” Narrated by Abu Dawood and Ahmad.
And Allaah is the Source of strength. May Allaah send blessings and peace upon our Prophet Muhammad and his family and companions.
From Fataawa al-Lajnah al-Daa’imah li’l-Ifta’, 23/318.