There's a very detailed guide on this site. It takes you step by step through flashing of the K810 Main and FS which are done in the same manner as you would if your reflashing. Only difference is that you use the K810 Main and FS files instead.
You then need to replace the light effects of the K810 with those of the K800. The files should be available on the net if you search.
Anyway, the link above explains it well, getting the Main and FS files may be the tricky bit. Good luck