Laravel, how to run update on table column using old value

I have a table of users with encrypted (not hashed) passwords.

I want to update rows that are using a certain encryption (not all rows are using this encryption)

I need to use the old value for the password, decrypt it, and encrypt it using a new encryption.

 $result = DB::connection('writer')
             ->table('users')
             ->where('pw', 'LIKE', 'oldEncryption')
             ->get();

 foreach($result as $r){
        $password = olddecrypt($r->pw);
        $newpassword = newencrypt($password);
        $r->pw = $newpassword;
 }

 DB::connection('writer')
     ->table('users')
     ->update($result);

heres the non working "pseudo code" any good approach for this?

This should get you started in the right direction.

$users = DB::connection('writer')
              ->table('users')
              ->where('pw', 'LIKE', 'oldEncryption')
              ->get();

foreach ($users as $user) {
    DB::connection('writer')
        ->table('users')
        ->where('id', $user->id)
        ->update(['pw' => newencrypt(olddecrypt($user->pw))]);
}

newencrypt and olddecrypt I'm assuming are your own implementations.