The hard fork will come into effect by the October 31st. Theres no plan to issue new coin. If somebody will do it, basing on the old network, the new coin might be issued but there are no guarantees that any exchange will support it.
with 4/5ths of all blake2b equipment NOT being an Obelisk machine and not being able to mine Siacoin after Oct 31st, 2018, well I'd be shocked if they don't run the old blockchain and call it Siacoin Classic or some such.
So the problem with an 'exchange' taking such a coin with that much hash will be likely moot.
I'd like to be wrong, but just can't see folk (big and small) sitting on that much equipment as 'doorstops' without doing the above.
Brad