What Is the Difference between a Soft Fork and a Hard Fork in Cryptocurrency Protocol Changes?

A soft fork is a backward-compatible protocol upgrade, meaning older, non-upgraded nodes will still recognize the new blocks as valid, though they cannot enforce the new rules. A hard fork is a non-backward-compatible change that requires all nodes to upgrade; if not, the network splits into two separate chains with different rules.

Hard forks are typically more disruptive.