No exchange that I know off will replace your BTC with BCC they either give you both or give you your BTC back please double check your wallets you probably still have your BTC + new BCC .
Poloniex and Coinbase will likely retain your BTC *BUT* they said they get to decide who the winning branch is. Coinbase said it doesn't support BCC, so it'd be awkward (=scandal) if they choose BCC. Poloniex was very ambiguous -they reserve the right to retire betaking your BTC and giving you BCC (calling "BCC" the new "BTC"). If they do that, they are out of business (but retired). Their statement are again not committing to doing anything in particular.