Have a November ATL to JFK to FCO main cabin awards ticket that I need to change because of a cruise cancellation. I would like to book a different date on a direct ATL-FCO flight that requires more miles.

Is the best way to do this to cancel the existing ticket and rebook, or try to get to customer service?
Are there any glitches or gotchas to watch out for in doing this? Their maze of current ticket rules seem to indicate the possibility that I can only "change" a ticket, but will lose the value (miles?) if I cancel it.
If I can cancel & keep miles, how long should it take for miles to show back in account?

Thanks,
C