I'm not sure this will work. When you copy the wallet.dat from the old wallet to the new wallet, i think there is a validation process of the wallet. The coins in the new wallet may just be rejected and not appear. And moving coins on the old chain can make things worse to get them on the new chain.
I can be wrong, i'm not a fork expert, it's not a situation we see often. The best is maybe withdraw small amout many times instead of one big.
It would be more easy if bittrex clone his wallet.dat into the new wallet and let people withdraw from this special wallet.
1. You're right, rejected.
2. This is what I think in first place, but what if some of trolls try withdraw their billions from preforket badget on bittrex. They will not check all users trade history (even 2sec script job) to eliminate trolls.
BTW. downloaded old wallet, synced and lost NJAs are back on exchange.. tried withdraw 250NJA (0.1%) and they are in new wallet/forked chain now =)