I know, that I still can track subscription state using web_order_line_item_id, BUT I am interested in reasons why id can change

Okay, finally I found some information: The field is_upgraded will be true also when cross-grading subscription of the same duration. In other words, is_upgraded = true is not actually an upgrade, but it’s changing subscription with a refund. (can't link site). SO remained question is: is_upgraded means, that part of price = (expires_date_ms - cancellation_date_ms) / (expires_date_ms - purchase_date_ms) is refunded, is this right?

Thank you very much for detailed answer!

Also I'm not sure about other APIs to track subscriptions. It would be very convenient to use a key like (app, user, sub) or (bundleId, _, productId) (didn't find any userId) and be able to get last transaction for this user in this app for this product for example