I don’t know how long it lasts or what the numbers are, but when a character gets a rework they have an increased sell-back rate. As you’ll recall, 3* Spider-Man was reworked a season or two back and Riri was also rerecently rereworked.
I’ve never understood the sell-back system, honestly. If they change a character and you now wish you hadn’t invested all of that ISO into them, shouldn’t you get to keep the character at the number of covers you have them at and just get back all of the ISO you plugged into them to use elsewhere?
That would be a little too customer-friendly. If they apply it evenly it’s got to be an option for everybody, and since a signficant chunk of the player base rosters every character (and sometimes multiples) they’d have to give away that free Iso to everyone just by saying “I don’t like this change”. You have to return the product to get your money back, or else there would be a massive Iso influx into the game after every rework.