I have found such information on documentation:
Upgrade. Someone purchases a subscription that offers a higher level of service than their current subscription. They’re immediately upgraded and receive a refund of the prorated amount of their original subscription. If you’d like people to immediately access more content or features, rank the subscription higher to make it an upgrade.
Also,
To identify whether a subscription has been refunded, look for the cancellation_date field in the receipt.
However, when looking at users that has upgraded subscriptions, where we get
"is_upgraded": "true"
There is no cancellation_date field, so the question is, has these users been refunded? If so, how do we calculate partial refunds without the date?