Please contact email@example.com with any refund requests clearly documenting your circumstances.
To be fair and use your analogy... your old car still continues to work as it did before.
We didn't just tune up the old car, we gave you a new one at 50% off the price based on your standing as a current car owner. The new car works on voice activation and costs a little more.
Your friends may want to continue driving your old car if they don't think the features in the new car warrant the price, which they can continue to do without issue.