The reason would be to change the reason for the weak password. How to do this?
If you read my previous answer, this should make things clear for you...
If you used a weak password to encrypt your private key, and there is any chance whatsoever this key encrypted with the weak password ever touched an online machine, you're basically best off creating a brand new paper wallet using a strong password to bip38 encrypt your key, decrypt your "old" private key, import it in electrum, fund your "new" paper wallet (emptying out the old one, mark it "compromised").
IF you are 100% sure your key encrypted with a weak password never ever touched an online machine and no copies were ever shown to anybody but yourself, you could potentially use the offline version of whatever tool you used to bip38 encrypt your private key to decrypt it, then re-encrypt the same key using a strong password. After double-checking the decrypted versions of both encrypted keys match, you should probably burn the old paper wallet (with the weak password). Only do this after verifying tough!
I'll try to give a basic example that should make things clear for youyou use any tool to create "MySuperSecretPrivateKey", the public key hash of "MySuperSecretPrivateKey" is "1MyFirstAddress".
Anybody who gets their hands on "MySuperSecretPrivateKey" can spend any unspent outputs funding "1MyFirstAddress". Luckily, you bip38 encrypted "MySuperSecretPrivateKey" into "MyEncryptedSuperSecretPrivateKey" so now, people need your password to decrypt "MyEncryptedSuperSecretPrivateKey" before they can rob you.
To bad you chose a weak password ("YourBirthDate").
You realise this, so you decrypt "MyEncryptedSuperSecretPrivateKey" using password "YourBirthDate", the output is "MySuperSecretPrivateKey".
You Re-encrypt "MySuperSecretPrivateKey" with a strong password ("AllRandomLetters"), the output is "MyStrongEncryptedSuperSecretPrivateKey"
HOWEVER....
decrypt("MyEncryptedSuperSecretPrivateKey", "YourBirthDate") = "MySuperSecretPrivateKey"
decrypt("MyStrongEncryptedSuperSecretPrivateKey", "AllRandomLetters") = "MySuperSecretPrivateKey"
Sooooo... IF ANYBODY, and i mean ANYBODY had POTENTIAL access to "MyEncryptedSuperSecretPrivateKey" at any point in time, he/she can easily brute-force "MyEncryptedSuperSecretPrivateKey" and find "MySuperSecretPrivateKey", it doesn't matter the slightest bit that you re-encrypted "MySuperSecretPrivateKey" with a strong password.