If a player is no longer allowed to cast a spell after completing its proposal (see rules 601.2a–d), the casting of the spell is illegal and the game returns to the moment before the casting of that spell was proposed (see rule 733, “Handling Illegal Actions”). It doesn’t matter if a rule or effect would make the casting of the spell illegal while determining and paying that spell’s costs (see rules 601.2f–h) or any time after the spell has been cast.
June 10, 2022 · Commander Legends: Battle for Baldur’s Gate
Some spells specify that one of their controller’s opponents does something the controller would normally do while it’s being cast, such as choose a mode or choose targets. In these cases, the opponent does so when the spell’s controller normally would do so.
October 4, 2019 · Throne of Eldraine
Casting a spell that alters costs won’t affect spells and abilities that are already on the stack.