Actually the one way cheapo PHX-FLL are often half the round trip fares thanks to the low fare competition. I was just looking at a CO PHX-FLL one way at $112 with a 2 week advance purchase.

I would recommend calling to cancel the flight saying you need to reschedule at a later date. Nobody cares what you do later, if the fare is worth more then the $100 change fee you can use the residual later.

p.s. We mileage runners know that by boooking two one-ways rather then a round trip yields double online booking bonuses.