when developers publish a change to any of a blockchain s functions one of two things happen a hard fork or a soft fork if the change is drastic enough to prevent a participant running the old version from participating in the blockchain s processes the change is considered to have created a hard fork in the blockchain conversely a change that allows old and new nodes to run is considered a soft fork 
in a hard fork event participants are faced with a choice they can update their software to the new version or not upgrade their software and use the old and now unmaintained blockchain reasons for a hard forkthere are several reasons or even events that cause developers to implement a hard fork for instance when the hack on the decentralized autonomous organization dao occurred the ethereum blockchain was forked by a nearly unanimous vote the fork rolled back transactions that siphoned off tens of millions of dollars worth of digital currency by an anonymous hacker the hard fork also helped dao token holders get their ether eth funds returned however some miners kept mining the old chain which was picked up by otc desks and given a price discovery order book poloniex listed this currency also and some of the ethereum community decided to keep this version and created ethereum classic 1