In some scenarios, it makes sense to program in hooks for upgrading functionality. Since contracts can call other contracts, you can just write a new contract a

In some scenarios, it makes sense to program in hooks for upgrading functionality. Since contracts can call other contracts, you can just write a new contract and point the old contract at the new functionality if the architecture is suited for it. Other than that, you basically write smaller, well-tested programs that don't irrevocably lock up people's assets.