How successful have you all been in upgrading on HP discounted tickets this year?

I am traveling from LAX to PHX. The discounted rates vary from $ 99 to $ 179 roundtrip(some of which are even refundable!). The Y fare is about $ 500 roundtrip.

Should I spring for the Y fare, just to guarantee my spot in FC? Or given the relative short duration of the flight, should I take advantage of the low fares, and try my upgrade luck at the gate?