Your sentence doesn't sound natural to me... It should read: "I'm not comfortable ABOUT changing a product without clear reasons." You could say: I'm not comfortable 'with' the new product. So: I'm not comfortable 'with' the direction the company is taking. I'm not comfortable with the new CEO. But: I'm not comfortable 'about' firing so many employees. I'm not comfortable 'about' developing so many new products at the same. time. It's better to use with+a thing (a noun) It's better to use about+an action (a verb) I cannot promise that this is always the case; and I can't guarantee that it is a fixed 'rule', but it seems to me that this is the general usage of these two words, in this structure. 🙂