The whole idea of blockchain is to run an immutable trustless ledger.

Allowing later modifications to older blocks would essentially make the whole idea pointless. It will then require "trust" in those who can change the historic blocks.

Simplified: a new block validates against the previous block, if a previous block gets changed, all subsequent blocks become invalid. So changing old blocks would also require rerunning the whole chain from that point on.

Technically this would only be possible if all "miners" (in case of steem witnesses) were in on it, but even then, copies of the unchanged blockchain would still be available. You would basically create a new chain all together.