If Singularity targets two units and one is Retreated before Singularity resolves, can the Singularity player retarget another unit?

Ruling: No, targets cannot be retargeted once chosen. When Singularity is played onto the chain, its two targets are locked and cannot be changed. If one target is Retreated by the time Singularity resolves, the other target still takes damage but no retargeting occurs.

Nuances:

• You can Retreat a unit in response to Singularity before it resolves (using priority)
• If you Retreat before target selection (in response to a reflexive trigger that creates Singularity), the opponent can choose targets after your Retreat resolves, potentially to their benefit