The only way to do this will be to use Tools -> New Config.
I would suggest that you start by using the option to retail current configuration and then return to the main tab to make the required changes. If you think parity is valid then you can tick the "parity is Valid" option to avoid an immediate parity rebuild, but if you do so I would recommend doing a parity check afterwards to make sure. The existing data drives will be left with their data unchanged, but keep the old drives intact until you are back up and running to keep a recovery option available.