AFAIK, none of the Bitcoin updates in the last few years is a "hard fork", that is, old nodes can still process valid blocks made by new nodes (though they migh

AFAIK, none of the Bitcoin updates in the last few years is a "hard fork", that is, old nodes can still process valid blocks made by new nodes (though they might accept blocks which new nodes would reject). Changing from proof of work to proof of stake would require a hard fork.