Will it be possible to import a private key for the legacy chain into the market client ? (Or into a new QT client which supports the post-fork upgrade) ?
I don't know if I'll get upgraded in time for the fork.
Your keys (for client) or wallet.dat (for Qt) will work after the fork too. You just have to load them in the wallets we released Nov 1st. But please, update before and leave your wallet running if you can. We would like to see 90% or more on the new protocol when we fork.