PHP form check if fields are empty if not change value in mysql table

Could someone help me with this code!!! So I’m trying to do a change user info part on my website, there’s everything working except the change username part. In this part I have to check if the newUsername is not taken and then change it (if its not already taken), but when I launch my code it doesn’t enter the (else if) statement after the (echo “username deja utiliser”). Can someone explain why it does that and give me a way to correct this problem

//change.php

$newUsername = $_POST['changeUsername'];
$newNom = $_POST['changeNom'];

$currentId = $_SESSION['id'];
$currentName = $_SESSION['name'];

//fonction that doesn't work
if(empty($newUsername)){
    echo "username is empty";   
    }else if ($stmt = $con->prepare('SELECT username from users where USERNAME = ? ')) {
    $stmt->bind_param('s', $newUsername);
    $stmt->execute();
    $stmt->store_result();
    if ($stmt->num_rows > 0) {
        $stmt->bind_result($checkUsername);
        $stmt->fetch();
        if ($checkUsername === $newUsername) {
           echo "username deja utiliser";
        }else if($stmt = $con->prepare("UPDATE users SET USERNAME = '$newUsername' WHERE ID = ? ")){
            $stmt->bind_param('s', $currentId);
            $stmt->execute();
            echo "les valeur on été modifier";
            session_regenerate_id();
            $_SESSION['name'] = $newUsername;
        }else{
            echo "les valeurs n'ont pas été modifier";
        }
    }
}

//function that works
if(empty($newNom)){
    echo "nom is empty";
}else if($stmt = $con->prepare("UPDATE users SET NOM = '$newNom' WHERE ID = ? ")){
    $stmt->bind_param('s', $currentId);
    $stmt->execute();
    echo "les valeur on été modifier";
}else{
    echo "les valeurs n'ont pas été modifier";
}

    //editProfile.php

    <form action="change.php" method="POST">
     
      <table>
        <tr>
          <td>Username:</td>
          <td>
            <label for="username"></label>
            <input type="text" name="changeUsername" placeholder="<?php echo $_SESSION['name'] ?>" id="changeUsername">
          </td>
        </tr>
        <tr>
          <td>Nom:</td>
          <td>
            <label for="nom"></label>
            <input type="text" name="changeNom" placeholder="<?php echo $nom ?>" id="changeNom">
          </td>
        </tr>
      </table>
    <input type="submit" value="modfier">
    </form>

Answer

Finally I found something that works. Is this code secure for SQL injection ?

//check if the username is taken 
if(empty($newUsername)){
    echo "username is empty";
}else if ($stmt = $con->prepare('SELECT username from users where USERNAME = ? ')) {
    $stmt->bind_param('s', $newUsername);
    $stmt->execute();
    $stmt->store_result();
    if ($stmt->num_rows > 0) {
        $stmt->bind_result($username);
        $stmt->fetch();
        if ($username === $newUsername) {
            $checkUsername = true;
        }
    }
}

//change the username
if (empty($newUsername)) {
    echo "username is empty";
} else if ($checkUsername != true) {
    if ($stmt = $con->prepare("UPDATE users SET USERNAME = ? WHERE ID = ? ")) {
        $stmt->bind_param('si', $newUsername, $currentId);
        $stmt->execute();
        echo "les valeur on été modifier";
        session_regenerate_id();
        $_SESSION['name'] = $newUsername;
    } else {
        echo "les valeurs n'ont pas été modifier";
    }
} else {
    echo "username deja utiliser ";
}