There's a BitShares console command 'wallet_change_passphrase'.
You can view help for this command with:
help wallet_change_passphrase
Change the password of the current wallet
This will change the wallet's spending passphrase, please make sure you remember it.
Parameters:
passphrase (passphrase, required): the passphrase for encrypting the wallet
Returns:
void
aliases: walletpassphrasechange
I never used it, so cannot comment further.
If you change the passphrase, make sure you can open your wallet with it.
You can access 'console' commands advanced menu option.
I tried linux GUI client a few times, but find console client more fun.
You can start BitShares console client (after you close GUI) running 'bitshares_client' file.
Type 'help' and you might get more reassuring experience. The GUI will catch up eventually.