The passphrase set during the fundraiser can not be changed. After you have entered the fundraiser details, some wallets will store your private key in a keystore file encrypted with a password. In this case the password can always be changed. If there is no built-in function for this you can always enter your fundraiser details again and create a new ...
I don't know about bakechain, but I can say your tezos-client command is incorrect.
It needs to look like:
./tezos-client import secret key my_account encrypted:edesk1XXXX
where encrypted:edesk1XXXX is the encrypted version of your key.
If using pytezos that would be:
>>> from pytezos import Key
Under the hood:
Base58 decode (with checksum)
Cut first 4 bytes
That's ed25519 ...
The passphrase used for fundraiser wallets contribute to the makeup of the secret key, so it's impossible to change it (these passwords aren't actually used for encryption). If you believe your wallet details may have been compromised, you should create a new key and transfer your XTZ.