We have designed an extensive upgrade protocol for the Internet Computer that is controlled by the NNS. Honest node operators (i.e., data centers) can be sure that their nodes will follow this protocol if they have done a correct installation. The upgrade protocol can also cover OS upgrades in addition to upgrading the replica software. Issues are resolved through NNS voting, and once approved, upgrades are automatically rolled out.
As for the hard fork, while nothing prevents individuals from starting a new Internet Computer by themselves, they would be unable to obtain the same key material that drives the NNS network on the Internet Computer, so they would have to choose a different public key. Furthermore, it will be next to impossible for them to hard fork with the identical state of the Internet Computer, because there is no way to obtain such data, even for node operators. Data privacy is taken very seriously.