Quote from WizardMN »Based on the last sentence of rule 114.6a, if you can't change all of the targets, then none end up being changed. In this case, that means that since you are unable to change all 4 targets, they all remain the same. Then, the spell will be countered on resolution since none of its targets are legal.
Thanks for directing me to that section of the rules. However, if I'm reading it correctly, it looks like 114.6d is the relevant rule:
114.6d If an effect allows a player to “choose new targets” for a spell or ability, the player may leave any number of the targets unchanged, even if those targets would be illegal. If the player chooses to change some or all of the targets, the new targets must be legal and must not cause any unchanged targets to become illegal.
That leads me to believe Segoth's response is correct, since Cooperate uses the text "choose new targets".
Can anyone confirm or refute?