When creating an altcoin base on bitcoin you generate a genius block private key which controls that chain.
I find this surprising, and I suspect that you are misunderstanding the altcoin, but since I pretty much ignore anything having anything to do with the thousands of scamcoin altcoins out there, I'm willing to accept that there probably are some such that the coin provides the users absolutely zero protection beyond trusting the coin's creator.
If the alt-coin is based on Bitcoin then it will follow Bitcoin's data structures. In bitcoin, the only thing to "control" is a user's address and wallet using your private keys to sign transactions. Transactions only transfer UTXOs and don't change anything in a historic block. Protocol changes ("control that chain") means changes to the consensus rules. Those generally require an upgrade of the client you are running as well as what the majority is running.
I think you are talking about Ethereum based coins. There you have the concept of contracts that can be over-written by the developer that deploys them to change to whatever they want.