why doesn't it work with redNand right after? redNand use the old sysnand?No and yes.
Upon formatting sysNAND, all your USBs linked to your Wii U will no longer work. You will have to reformat the USB to your Wii U after unlinking the NANDs.
This means that all your games and saves will be deleted. If you really want to format your sysNAND, backup your saves first.
However, once you reformat your USB, it will work on redNAND.