Is it possible to restart the Bitcoin blockchain from the point where the block was saved? For example, now, as of January 1, 2024, block No. 823.881 has been mined. And I want to restart it from block No. 800,000. Is this real or not?
Yes, such a scenario, though difficult to implement could be considered if the total computing power of the attacking (ganged together ) nodes and the nodes they attack exceeds the hashrate of the remaining nodes.
Therefore, in order to prevent their encirclement and isolation, nodes are recommended to carry out a number of activities, namely, increasing the number of connections, selecting peers randomly and changing them from time to time, monitor relevant traffic, and so on.