I had luck calling in to make the rebooking.
I booked J saver for JFK-FRA-SIN-MLE round trip. The SIN-MLE round trip flight got cancelled about 4 months after and I got the email with the automatic rebooking. After calling in, the agent was able to make adjustments even though I didn't see those saver availability online. I had the chance to get a later MLE-SIN flight and ended up with MLE-SIN-JFK, which didn't have any reward availability by that time.