cr:1.8.5f

If an effect modifies the target of a targeted effect, only legal targets for the targeted effect may be selected as new targets, unless otherwise specified by the modifying effect. If there is no other legal target for the targeted effect, then the target remains unmodified.

Example: Taipanis, Dracai of Judgement has the text “The first time each turn another hero becomes the target of a source that would deal lethal damage, you may discard a red card. If you do, choose new targets for that source,” which can modify the targets of an effect but can only select legal targets as per the original targeted effect. Taipanis cannot change the target of a hero’s attack to be their own hero, because attacks can only target opposing living objects.