Whether that is a good deal is a question that only you can answer; a "good deal" is subjective and it may sound good to some but not others.

Use the QCalculator to see how many Avios are required for upgrade - it depends on the fare bucket.

I am not sure about your last question - assuming that you mean cancel your booking for free if the flights change, I think it depends on how much the flights change. (They may not allow it if it's a 10 min change to the flight times...)