Canceling it doesn't do you much good. Then you'll have a credit, but be forced to buy an expensive one way home. Book it for as far out as you can without running afoul of immigration regulations. Then when you know when your return will be, move the date up for the $150 + fare difference. Just do it once you are pretty sure of the date. Then you still have round trip pricing, only 1x $150 change fee, and you're not running afoul of visa restrictions.