This is something we've not seen before - usually CME give a day or two notice to allow traders to deposit the additional margin, however with this announcement, the consequences could be chaotic trading as positions are closed out automatically before margin can be added!
It is impossible to tell whether markets will spike upwards or fall dramatically, or neither of these, as the positions that are forced to close may be short or long
Instinctively, we would suggest that these kind of chaotic conditions would mean a big fall at the open, however in our view it is likely that many short positions will be "underwater" after last weeks' trading, so we may see a lot of short covering triggered by this decision. Ultimately, we're just speculating - Bonfire Night may have been last night though the fireworks may just be about to begin in the markets.....