upgrade-cairo-contracts

Starknet separates contract instances from contract classes. A class is the compiled program (identified by its class hash); a contract is a deployed instance pointing to a class. Multiple contracts can share the same class.

Upgrading a contract means replacing its class hash so it points to a new class. The contract keeps its address, storage, and nonce — only the code changes. This is fundamentally different from EVM proxy patterns:

| Mechanism | replaceclasssyscall swaps the class hash in-place | Proxy delegatecalls to a separate implementation contract | | Proxy contract needed | No — the contract upgrades itself | Yes — a proxy sits in front of the implementation | | Storage location | Belongs to the contract directly | Lives in the proxy, accessed via delegatecall |