After this card's [On Play] effect was used to prevent a level 4 or lower Digimon from attacking, the Digimon digivolved and became level 5 or higher. Can this Digimon attack?
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Official Ruling
Yes, it can attack.The effect reads ""all of your opponent's level 4 or lower Digimon,"" therefore the effect will no longer affect Digimon that become level 5 or higher after the effect activates. On the other hand, Digimon will be affected by the effect if they become level 4 or lower or a level 4 or lower Digimon is newly played on the battle area after the effect activates.
Source: EX4-021 GreyKnightsmon