I have read that it is not possible to reverse transactions already added to the blockchain but I do not understand why that is not possible in a 51% attack.
If an attacker was to change the information of a past transaction, the hash of that transaction would change. This would result in a different Merkle root for that specific block. Since the attacker controls 51% of the hash rate, he would have enough computational power to find the new Merkle Root and the nonce this new block in order to produce a valid proof of work. Then the minder could broadcast the proof of work and approve it himself since he controls the majority of the network. In that way he could change the contents of a past block.
My logic above is definitely flawed somehow. Could someone please explain why a 51% attack cannot change past transactions?
Does the digital signature somehow play a role in this? If yes how?