It totally depends on the contract. In theory, yes, all contracts are immutable. But there are various architectural patterns which can be used to make them less immutable - such as proxy pattern which keeps part of the contract in another contract and the other contract only references the other contract. That way the reference can be updated to point to a newly-deployed contract instead of the old contract, and thus changing the contract's functionality.

So if a contract supports such functionality some of its aspects can be modified.