When do cards with targeting effects (like 'Kill a unit at a battlefield') choose their targets, and can targets be changed if the original target becomes invalid?

Ruling: Targets are chosen when the card enters the chain (when you play it). You cannot change targets after they are chosen.

Nuances:

• If the original target becomes invalid (e.g., no longer at the battlefield) before resolution, you cannot choose a new target even if other valid targets exist