Hard fork really happens when more than 75% of nodes/miners/hash power successfully bypass the set rules of protocol, in our case the protocol states blocks larger than 1MB are not valid but if enough numbers of nodes/miners accept any block larger than 1MB nodes validating and not enough nodes/miners could compete and create a normal block and not enough nodes accept and validates that normal block to be added to previous normal chain of blocks then the network has practically experienced a hard fork (change of the set rules.
Right now if enough of nodes/miners running Core software switch into BU then they have gone rogue.

Every software/program update/upgrade is basically a hard fork but in their coding everything overwrites and deletes the old files but blockchain can not be overwritten and deleted and must continue to function.

After any splits people still can run Core wallets of older versions like 0.12/0.13 as long as those versions are compatible and didn't change the set rules(protocol) they can keep mining their own chain but it will depend on users and especially exchanges if they will list and give the BTC ticker to it or name it something else(an altcoin).