get shield item

shield.addPropertyOverride(new ResourceLocation("yourmod", "blockheld"), new IItemPropertyOverride() {
    @Override
    @SideOnly(Side.CLIENT)
    public float apply(ItemStack stack, World worldIn, EntityLivingBase entityIn) {
        if stack == entity's main hand
            checkhand = offhand
        else checkhand = mainhand

        if (checkhand holding a block)
            return 1;
        else return 0;
    }
});



in the main shield.json

{
    "parent": "builtin/entity",
    "overrides": [
        {
            "predicate": {
                "yourmod:blockheld": 1 // this the resourcelocation you made above. The JSON tests for a value greater than or equal to what we specify here. Predicates tested from top to bottom
            },
            "model": "yourmod:item/shield_with_block_held" // give it another item json
        }
    ]
}

in shield_with_block_held

{
    "parent": "builtin/entity",
    <add your transforms>
}