Nodes set the rules, not miners
If the majority of miners mine blocks against consensus rules, as defined by nodes, nothing happens, as mined block, even if the longest chain is produced, are not relayed trough the network as rejected by the nodes.
The only option a so-called 'node' has when it encounters a chain it doesn't like is to orphan itself from that chain. And hope it can find another it is happy with. If the miners en masse move to a new implementation, each and every so-called 'node' merely fucks itself individually.