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You are correct - it doesn't take the points until you are actually upgraded. Make sure you still have enough points that day, and don't change your PIN between the request and the day of departure...From what i understand...
Depending on your fare type, it is possible for 72000 points (econ -> Bus). You must register for the upgrade before hand though, and you will only get charged the points if the upgrade goes through.
I haven't tested this though, would be interested in others' thoughts.