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Could miners intentionally change the output address or reduce the output value before forming a block

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A block must have exactly one coinbase transaction as its first transaction. This transaction may collect the transaction fees and create new coins according to the subsidy. It can assign those funds to one or multiple outputs. If the block includes any segwit transactions, it also has to include an op_return output with a witness commitment.

There have been multiple instances in which miners have collected less funds than they would have been allowed to. For example block 501,726 failed to collect any mining reward at all.

The only time when the coinbase transaction can be changed is before a block is found. Miners routinely amend the coinbase's extranonce during the mining process to add more entropy to their block candidates. Once a block template produced a valid block, the coinbase is fixed, since changing any byte in the block would change the header and the header's hash would no longer fulfill the difficulty requirement.

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To my understanding, a miner will define the output address for the payout of the coinbase transaction prior to solving the hash and sharing the new block. Changing the output value of the coinbase transaction would break protocol rules and the block would be invalid and not added on top of the chain.

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