Lets say there is a miner underground movement to recompile the client with 8MB block limit.  They can do it, and they don't even have to tell anyone.   However, the first ones to do it are risking it, because their big blocks will probably not end up in the longest chain. 

However, if more than 50% do it, we get a hard fork, and small block miners are forced to switch.

And, believe me, exchanges would be first ones to switch, as they have most to lose.