A note about this: before the release of Ardor this 2017 (including IGNIS as first Ardor child chain) it is recommended that the myNXT wallet users export a backup of their wallet, so besides the web wallet access, everyone has a copy of the actual Nxt passphrase that unlocks the account/s held in the wallet.

The reason is that, at Ardor launch, the amount of ARDR assets in every Nxt account will produce the same amount of Ardor main chain tokens in the new platform, and the amount of NXT in every Nxt account will produce an amount of IGNIS (rate to be decided by the development team) in the first Ardor child chain.

I understand that these tokens will be assigned to the Ardor accounts with the same private key as the source Nxt account, so in order to log in to the Ardor platform and see the tokens you earned in Nxt, you'll need the Nxt passphrase for the account - which is not used in the regular myNXT wallet operation. In the myNXT wallet, passphrases are stored encrypted with the users master passwords, so in order to get the actual passphrases you need to export your myNXT wallet. Steps to do that can be found here: https://nxtforum.org/mynxt-info/accessing-your-mynxt-wallet-accounts-in-the-nxt-client/