Converting Diverter-Tee System to Loop
The system you describe is a "monoflo" system. They work pretty well when installed correctly. It sounds as though yours is installed correctly. If you want individual control in each piece of radiation you could add rad valves to each radiator piece. If you are not well versed in this, I would seek professional help so you don't change something that is very costly to fix. Add to your original question so that we all have an idea just what you are trying to accomplish. There are any number of changes and improvements you can do. None are easy or cheap or quick. Please indicate what you are trying to do.