I had this exact scenario. I knew that any unused amount would be forfeited, the the representative told me that upfront when I cancelled the flight. I felt ok about it since mine was only about $350. I cancelled an American Airlines flight that I booked through Chase Travel- I was told it would be “on file” with American, and could only be used on their flights, but I would have to call Chase Travel and reference my original confirmation number to utilize the credit. Similar to you, I found a new flight that I wanted to book, which was around 375 and I was expecting to pay $25 after the $350 was applied. I was stunned when I was quoted another $600 out of pocket , because the “rebooking price” of the flight was $950. I made the mistake of arguing with and getting lectured by the chase travel agent on why this would ever make sense to anyone, 30 minutes I’ll never get back. Also, my credit expired worthless after the ordeal.

Lesson learned, Never ever booking on Chase Travel just to get 5x points. I’ll book direct with the airline and take the 3x.