> If the developer introduces a major change, it'll fork the chain. Everyone on the old version of the software will see one chain, everyone on the new version will see the new chain.
Why? Can't the system keep using the same chain, or is this just convention? If any software change required a fork, you couldn't do even the most trivial of bug fixes (say, fix a typo in the UI, or whatever). But I notice you said "major change"; so who decides what's "major" and what isn't? On which grounds?
It's not convention. If it's possible to create a transaction that one version thinks is valid, and the other doesn't, someone will create such a transaction, it will be included in the chain that accepts it if that side has the majority of the hash power, and the other one will reject it. Thus you will have 2 chains (not to be confused with 2 Chainz).
The definition of "major change" in this case is a change that changes the definition of a valid block or transaction.
Why? Can't the system keep using the same chain, or is this just convention? If any software change required a fork, you couldn't do even the most trivial of bug fixes (say, fix a typo in the UI, or whatever). But I notice you said "major change"; so who decides what's "major" and what isn't? On which grounds?