Then it isn't a soft fork, and it isn't backward compatible. The network splits into "original bitcoin" and "new protocol trying to call itself bitcoin". The only way that the new protocol can "win" is to convince nearly everyone (exchanges, merchants, consumers, investors, etc) to use their new "stealCoin" protocol instead of the secure "Bitcoin" protocol.
You can check Pieter's segregated witness proposal video here
https://www.youtube.com/watch?v=fst1IK_mrng#t=36mIt is a large change to bitcoin protocol (in fact changed pretty much everything in bitcoin) but still can be implemented using a soft fork, means backward compatible. I'm still scratching my head of how this is possible, but I guess it is using a technology similar to master coin, they can make the new design totally invisible to the old client, maybe embedded some key data in some trivial field, so the old client feels that nothing has changed