If you have enough points to cover both bookings, I think online would work. That would permit you to make sure you have the new room before giving up the original room and restoring the points to your account.
I'm not sure how you would move points from one reservation to another online without risking losing what you already have. That is where I think a phone call would be necessary.
WDW Visitor since 1973, DVC member since 2000
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