If I play Singularity and my opponent reacts with Retreat, can I change targets to hit their other units?

Ruling: No, you cannot change targets. You choose targets when you put the spell on the chain and cannot change them later.

Sequence:

• Choose targets when putting Singularity on the chain
• Opponent reacts with Retreat on one of your targets
• The retreated unit is no longer a valid target and the spell fails to hit it
• The other original target still takes damage

Nuances:

• If one target becomes invalid (like through Retreat), the spell still resolves against remaining valid targets