Will not work if your windows 10 license is OEM as it's a major hardware change(MB+CPU). Run "slmgr.vbs /dlv" without quotes to check if it's not. Otherwise, re-activation of your windows 10 requires either its product key or use your MS account.
Might go blue screen of death due to previous installation of would be conflicting drivers. Some lost of efficiency if your previous system is not UEFI compatible or GPT partitioned.
As for clean install, a backup of your document folder and the Appdata folders(optional as they contain previous configuration data for installed apps or games, even if you don't reuse, it will be recreated once you install said apps or games - local, locallow, roaming) is generally enough. If you use MS account, it will even resynced back to previous windows settings. Specialized software where you configure to use custom folders to store stuff, backup those custom folders if you plan to use that software. Backup can be burn to disc, copy to usb stick, or upload to cloud service or own NAS, or another storage drive like D:.