function get_user_total_purchased_items( $user_id = 0 ){
    global $wpdb;

    $customer_id = $user_id === 0 ? get_current_user_id() : (int) $user_id;

    return (int) $wpdb->get_var( "
        SELECT SUM(woim.meta_value)
        FROM {$wpdb->prefix}woocommerce_order_items AS woi
        INNER JOIN {$wpdb->prefix}woocommerce_order_itemmeta AS woim ON woi.order_item_id = woim.order_item_id
        INNER JOIN {$wpdb->prefix}posts as p ON woi.order_id = p.ID
        INNER JOIN {$wpdb->prefix}postmeta as pm ON woi.order_id = pm.post_id
        WHERE woi.order_item_type = 'line_item'
        AND p.post_type LIKE 'shop_order'
        AND p.post_status IN ('wc-completed')
        AND pm.meta_key LIKE '_customer_user'
        AND pm.meta_value LIKE '$customer_id'
        AND woim.meta_key LIKE '_qty'
    " );
}

add_action( 'woocommerce_order_status_completed', 'change_role_on_purchase', 20, 2 );
function change_role_on_purchase( $order_id, $order ) {

    $user_id   = $order->get_customer_id();
    $user      = new WP_User( $customer_id );
    $purchases = get_user_total_purchased_items( $customer_id );

    if( $purchases >= 50 &&  $purchases < 100 && ! in_array( 'bronze_user', $user->roles ) ) {
        // Remove role
        $user->remove_role( 'subscriber' );

        // Add role
        $user->add_role( 'bronze_user' );
    }
    elseif( $purchases >= 100 &&  $purchases < 200 && ! in_array( 'silver_user', $user->roles ) ) {
        // Remove role
        $user->remove_role( 'bronze_user' );

        // Add role
        $user->add_role( 'silver_user' );
    }
    elseif( $purchases >= 200 && ! in_array( 'gold_user', $user->roles ) ) {
        // Remove role
        $user->remove_role( 'silver_user' );

        // Add role
        $user->add_role( 'gold_user' );
    }

}

Anyone have any ideas why its not working?

If you add some debbuging statements, do things proceed through your ifs properly?

if( $purchases >= 50 && ! in_array( 'bronze_user', $user->roles ) ) {
        // Remove role
        $user->remove_role( 'subscriber' );

        // Add role
        $user->add_role( 'bronze_user' );
    }
 if( $purchases >= 100 && ! in_array( 'silver_user', $user->roles ) ) {
        // Remove role
        $user->remove_role( 'bronze_user' );

        // Add role
        $user->add_role( 'silver_user' );
    }
 if( $purchases >= 200 && ! in_array( 'gold_user', $user->roles ) ) {
        // Remove role
        $user->remove_role( 'silver_user' );

        // Add role
        $user->add_role( 'gold_user' );
    }

Hope it does helps.