Smart Contracts are immutable: they cannot be changed after deployment, only replaced. This goes along with the original idea of a trusted blockchain environment, but makes it difficult to maintain and upgrade them.
To mitigate these issues, EIP-2535 proposes a sophisticated proxy pattern, which allows the addition, replacement, and removal of functions and virtually removes any constraint on the code size; and an innovative storage layout, which is collision-resistant and very flexible.
In this article Cartesi's developer Guilherme Dantas dives into details of EIP-2535 “Diamonds” and its upgrading features: https://link.medium.com/KyzboqVXlsb
