Masternodes reject them if they don't comply.  That's why we watch the % of masternodes that update when we have a spork.  If Evan pulls the switch, and we have 90% compliance on the network with the latest wallet, then miners that don't update have their blocks rejected.  Only a block from the right version passes.  (well, there is a chance that a non-updated miner will hit one of the 10% masternodes but then we turn off the spork and try again.  When we take control of the blockchain (usually on first try) we have it.

So what I said was true for Dash, I just thought it might be true for Bitcoin as well, at least on the level of all nodes should agree.  In Dash, Masternodes hold the keys to the door.