In case it was not clear, in a 51% attack the attacker changes the "tip" of the chain not some random blocks deep into the chain. keep in mind that what we have in Bitcoin is a block-chain and if anything, even a single bit is changed in any old block, that block and the whole chain after that block become invalid.
An attacker is able to replace older confirmed blocks because with more than 50% they will eventually catch up and surpass the honest miners with a new longest chain.