user43565: Say, I want to erase the record of a transaction by changing a block. Can I go back to that block and find a new block and nonce combination that makes the same hash? 
Is it a case that finding such a new combination is computationally hard or is it actually impossible?

user3697: One of the main properties of a hash algorithm is collision resistance. If you would find an identical hash with two different datasets, then we would speak of a hash collision.
This is extremely unlikely to happen. The mathematical chance that a sha256 collision would happen is smaller then the actual chance that the universe will implode in this moment.
https://crypto.stackexchange.com/questions/24732/probability-of-sha256-collisions-for-certain-amount-of-hashed-values

43565: <p>Say, I want to erase the record of a transaction by changing a block. Can I go back to that block and find a new block and nonce combination that makes the same hash? </p>

<p>Is it a case that finding such a new combination is computationally hard or is it actually impossible?</p>
 3697: <p>One of the main properties of a hash algorithm is collision resistance. If you would find an identical hash with two different datasets, then we would speak of a hash collision.</p>

<p>This is extremely unlikely to happen. The mathematical chance that a sha256 collision would happen is smaller then the actual chance that the universe will implode in this moment.</p>

<p><a href="https://crypto.stackexchange.com/questions/24732/probability-of-sha256-collisions-for-certain-amount-of-hashed-values">https://crypto.stackexchange.com/questions/24732/probability-of-sha256-collisions-for-certain-amount-of-hashed-values</a></p>
