I upgraded LAX -> BOS for 291. I am little surprised to see that ticket price is not changed and if i try to change flight only original fare is available to apply. Does it mean that upgrade is not credited if i cancel?

I better not cancel then. I feel then it make sense to only buy upgrades close to flight. I am also within 24 hours of booking. Any one know what happens to upgrade paid value if canceled without 24 hours.