If a Digimon affected by this card's [On Play] [When Digivolving] effect later gains two or more digivolution cards, will it be able to attack and block?
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No, it will not. This effect targets one of your opponent's Digimon with one or fewer digivolution cards and gives it a "can't attack or block until their turn ends" effect. After the effect is given, it remains on the Digimon even if its digivolution cards increase.
Source: EX12-026 Shellmon