Create a full backup of your HHD. You can back it up to an external disk or something similar. You can try using a free program like Veeam Agent for Windows. It's a very good, free backup program.
After that you can remove the old HHD, install new SSD. Install fresh copy of Windows 10, reformat HHD and readd HHD to system as a secondary drive. You than can then recover from backup media to either disk of your choosing.
Another option would be to remove the old HHD, install the new SSD with Windows, then add the old HHD via usb adaptor to the system and manually copy off data you need to the new SSD, then format the HHD and manually reinstall as secondary disk in system. Then copy whatever data back to the HHD after its been formatted.
In either case, the old HHD should be reformatted to ensure there is not dual booting issues and to remove old, no longer used partitions.