It seems no other choice but export without password.
It seems need to be online and synchronize again to re-export key without password, but it is so slow now
. Is there any deadline /cut off date for move to electrum?
No, you don't need to be online or synchronised... you can be completely offline and use "tools -> export private keys". In fact, I recommend doing it this way.
- Get offline
- Export keys from MBC with NO password
- import keys into Electrum (File -> New\Restore -> Import Bitcoin addresses or private keys -> copy/paste private keys, one per line -> Set a strong password on the wallet file)
- (Optional) write you private keys down on paper, so you have an offline copy.
- Delete the exported key file.
- Go online and sync Electrum.
For added security, you could consider doing a lot of that with a Linux Live CD/USB boot disk that has networking disabled and no persistent storage. That would help minimise possible exposure of your private keys.
Also, there is no "deadline"... the BCH (and BTG) will sit on those keys until it is moved by someone with access to the keys.
For sake of security, will move BTC to keep key wallet, but about BCH?
If there is no password, anyone obtains this key can claim BCH based on it?
Yes, I would highly recommend moving the BTC as soon as you can... move them to a completely new wallet... and possibly your BCH too (you can use ElectronCash to get your BCH at the same time that you use Electrum)
Yes, if the file is in plaintext, anyone who gets access to that file will have access to your private keys and theoretically any BTC, BCH or BTG that has not been moved from the key.
NOTE: currently there are no decent wallets for BTG except the BitcoinGold Core wallet that requires a full blockchain download.