but I've heard that airlines can cancel your miles account.
In Peru, we missed a leg of our flight from Lima-Cusco. I asked if we could just book another one and they said if we did, they would cancel the rest of our trip (Cusco-Puno, Puno-Lima, Lima-SLC (via Houston, I think). So, we had to wait around for open seats on one of the Lima-Cusco flights which just replaced the one we missed.