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Use this Command in a repeat. Command Block(1.13+)

/execute as @a[nbt={Inventory:[{Slot:102b,id:"minecraft:iron_chestplate"}]}] run replaceitem entity @s armor.chest minecraft:chainmail_chestplate 1

However this will only work once the Iron Chestplate is placed on the player's armor slot.

Use this command in a repeat Command Block  (1.13+):

/execute as @a[nbt={Inventory:[{Slot:102b,id:"minecraft:iron_chestplate"}]}] run replaceitem entity @s armor.chest minecraft:chainmail_chestplate 1

However, this will only work once the Iron Chestplate is placed in the player's armor slot.

Try Pasting This Command In A Repeat Command Block(1.13+)

/execute as @a[nbt={Inventory:[{Slot:102b,id:"minecraft:iron_chestplate"}]}] run replaceitem entity @s armor.chest minecraft:chainmail_chestplate 1

However This will only work once the iron chestplate is placed in the Player's armour slot

Use this Command in a repeat. Command Block(1.13+)

/execute as @a[nbt={Inventory:[{Slot:102b,id:"minecraft:iron_chestplate"}]}] run replaceitem entity @s armor.chest minecraft:chainmail_chestplate 1

However this will only work once the Iron Chestplate is placed on the player's armor slot.

Try Pasting This Command In A Repeat Command Block(1.13+)

/execute as @a[nbt={Inventory:[{Slot:102b,id:"minecraft:iron_chestplate"}]}] run replaceitem entity @s armor.chest minecraft:chainmail_chestplate 1

However This will only work once placed in the Player's armour slot

Try Pasting This Command In A Repeat Command Block(1.13+)

/execute as @a[nbt={Inventory:[{Slot:102b,id:"minecraft:iron_chestplate"}]}] run replaceitem entity @s armor.chest minecraft:chainmail_chestplate 1

However This will only work once the iron chestplate is placed in the Player's armour slot