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.