If the majority of nodes agree that for all blocks after block #N (e.g. 400000) the only valid block must contain only the coinbase transaction with zero block reward, and with a alt-block hash attached to the coinbase message, where the alt-block hash is the hash of the block in blockchain 2.0 (transmitted separately), then it is technically only a soft fork because the set of valid blocks accepted by new nodes is a strict subset of that accepted by old nodes. The old nodes will be forced to accept a perma-frozen blockchain where no new coins are generated and no transactions will confirm.