If you want to increase the speed of your iMac, just replace internal HD with SSD, you don’t need to configure or to replace blade (PCIe SSD). You need to install macOS on the internal SSD (the new one). Fusion will not be configured automatically, there is only one way to configure it again and that you can do from Terminal (Internet Recovery Terminal), just type diskutil resetFusion and confirm with “Yes”. Before this procedure, be sure that you have backup of all of your data, because all data will be erased after resetFusion.