I am trying to remember back, but I think I dropped the df cold turkey with DS around 7 mo. He seemed to be taking less & less, so I just skipped it one night, and with no ill effects. I'd suggest trying it one night, what have you got to lose? And if that doesn't work, your idea of moving it earlier and earlier is a good one too....I'd go 15 min increments or so every few days.
good luck & let us know!