Answered by Shaykh Yusuf Weltch
Question: If I begin a make-up fast, then change my intention to a voluntary fast, does that affect the type of fast?
Answer: Jazak Allah Khairan for your question.
Your change of intention, after having started a fast, will not harm or alter your fast, whatsoever. The original intention upon which the fast was started will remain.
“…the fasting person, who is making up an obligatory fast, if after having started the fast, he intends to start a different fast, it would not harm (the original fast).” [Maraqi al-Falah]
Allahu ‘Alam
[Shaykh] Yusuf Weltch
Checked and Approved by Shaykh Faraz Rabbani