• Soft forks have been used to implement new and upgraded functionalities that are backwards compatible.
• There is no requirement for all nodes to be upgrade and agree on the new version.
• Regardless the updated users and the non-updated users could still interact with each other.

All the nodes need to agree and once done ALL previous blocks are invalid.

• Old rules vs New rules
• A forceful method to update the core software & its foundational protocol of the Blockchain to invalidate (or vice-versa) the few previously mined Blocks in the Blockchain.
• This is generally done to recover the Blockchain from drastic damage or attack.
• It is not backward compatible.