Such a number of exchanges are ready to provide support to ethereum, this clearly means that even if a bad situation occurs, they will all try to soften it!
All exchanges and services will do, it's logical, they don't have another option either. I mean, this is not a fork in which someone is going to create a new coin and they have to decide which one will support, this is just an upgrade and we are talking about Ethereum, not an unknown coin. I guess they will disable deposits and withdrawals before and after the fork, until everything is safe.