Hashing algorithms are one-way i.e. They cannot be reversed unlike Encryption-Decryption algorithms.
MD5()
is a hashing algorithm , so is SHA-1 / SHA-256;
It is good to know that you have stopped using MD5()
and moving to SHA-256()
Solution to your problem.
First run a query in your users table like this
UPDATE users_t SET PASSWORD=SHA2(PASSWORD,256) WHERE 1=1
Now all your MD5
Hashed passwords will be converted to SHA256
Hashes.
However, you cannot stop using MD5
on your login/register pages.. You have to change your checking logic like
$pass = $_POST['password'];
$hashedpass = hash('sha256',md5($pass)); // Now you need to insert/verify