Saiha is a very serious defect. The only way a sword can retain much, if any, value if it is retempered, is if it is signed with an important and unusual signature, or if it was retempered by an important smith because it was made originally by an equally or more important smith. At the National Sword Museum in Tokyo, years ago, I was shown a sword that was retempered, that had the only dated signature of an important and early smith, and that had a Juyo paper.
This is very rare. An unsigned sword that might be koto and might be Mihara school and is retempered isn't worth much: nothing anyone wants to collect.
Grey