One AMD Moderator and a couple of Users here at AMD Forums says yes you should do a clean Windows install after upgrading to a Ryzen Processor on the same motherboard.
My personal opinion is first install the new Ryzen Processor with your current Windows and do a CMOS Clear on your Motherboard to reset the BIOS to recognize the new Processor.
If you start having problems with Windows then do a Clean Install and see if that fixes your Windows problems but if Windows is working fine and recognizes the new Processor than it is your decision if you still want to do a Clean Windows installation.
By the way, Make sure your current RAM MEMORY modules are listed under your Motherboard's QVL List for compatibility for Matisse (3rd generation processor) list.
