IRC has a number of "upgrade" events
giftpaidupgrade / anongiftpaidupgrade and one related to "prime to real" that I don't have the tag/msg-id for
A "subscriber upgrades" topic would be useful, if this doesn't fall under "sub notifications" - https://twitch.uservoice.com/forums/310213-developers/suggestions/42008104-please-provide-an-event-sub-that-emits-sub-notifi
I could see a benifit to having this as a seperate topic, or tied into the "notifications" topic
We are investigating adding an EventSub subscription type for sub upgrade events.