This assumption is wrong, because you cannot insert blocks in the middle, and reuse other blocks. If you change anything in a block, then all blocks that were created later, are invalid, and you have to change them as well. So, for example, if you mine block 123, then you also need to mine block number 124, 125, and so on, up to the latest block (at the moment of writing, 783849). So, if you want to change anything in block 123, then you need to overwrite the whole chain, from that block, to the latest block, re-mining block 123 alone will be rejected, because each block has a field called "previous block hash", so you have to change the whole chain after that block.