I'm certainly no expert on this either but let me explain my thought process on this...
If you make a withdrawal from Bittrex, then open an old version of the wallet and let it sync the withdrawal from Bittrex should show up since they're on the same fork. At the same time, the old wallet is going to mint millions of coins that are invalid on the new fork, and all of this will be stored in the wallet.dat file.
Once everything is synced and the withdrawal from Bittrex (valid coins) and the minted POS coins (invalid coins) are in the wallet.dat file, and you open up the newest wallet, the valid coins should show up in your new wallet and the invalid coins should be rejected. That's the whole point of the hard fork, right?
But there is not CoinControl on exchange like in the wallet, where I'll be able to chose valid coins (from addresses before fork) to withdraw. Bittrex can send me my balance from address which is full of PoS coins from trolls/NJA assassins. I bought all my NJAs, not mined and sent to Ex. But I was able withdraw some NJAs (250+150) to new wallet. What I don't understand is, how new wallet/forked blockchain know that someone send these NJA/make transactions in invalid chain. So I thought that chains are mixed because of new and old wallet version.h (NOBLKS_VERSION_END)/same root NODE/same diff retarget (I don't know if it was changed during hard fork), but when i tried found block hashes solved by p2pool (on good fork) in blockexplorer (on same fork like Ex.) I can't find any. I want my NJAs or clarify the mystery.. both have same value for me (no I will not sell 250k for explanation

)