One-time update: Set user_nicename to a safe "first-name-last-name" format for existing users.

my_update_existing_user_nicenames.php

/**
 * One-time update: Set user_nicename to a safe "first-name-last-name" format for existing users.
 */
function my_update_existing_user_nicenames() {
	// Only run for admins to avoid performance hits or unauthorized access.
	if ( ! current_user_can( 'manage_options' ) ) {
		return;
	}

	// Add a transient so we only run this once.
	if ( get_transient( 'my_updated_user_nicenames' ) ) {
		return;
	}

	$users = get_users();

	foreach ( $users as $user ) {
		$first = get_user_meta( $user->ID, 'first_name', true );
		$last  = get_user_meta( $user->ID, 'last_name', true );

		if ( empty( $first ) || empty( $last ) ) {
			continue;
		}

		$desired_nicename = sanitize_title( $first . '-' . $last );

		// Skip if nicename already matches or if it would conflict with others.
		if ( $user->user_nicename === $desired_nicename ) {
			continue;
		}

		$base_nicename = $desired_nicename;
		$counter = 1;

		while (
			username_exists( $desired_nicename ) ||
			get_user_by( 'slug', $desired_nicename )
		) {
			$desired_nicename = $base_nicename . '-' . $counter;
			$counter++;
		}

		// Update user_nicename directly.
		wp_update_user( [
			'ID' => $user->ID,
			'user_nicename' => $desired_nicename,
		] );
	}

	// Set a transient to mark it as done for 1 year.
	set_transient( 'my_updated_user_nicenames', true, YEAR_IN_SECONDS );
}
add_action( 'admin_init', 'my_update_existing_user_nicenames' );