It is not typical, but not uncommon for a katana or wakizashi to be re-tempered if it was burned in a fire or had the kissaki broken
It needs to be done by an expert. Please don't try it yourself. It drastically changes the curvature and also will have a different hamon from what the original swordmaker did. Yes, Re-tempering makes the blade far less valuable in the Nihonto market. Any serious collector can spot a re-tempered (saiha) blade quite easily.
Rich
http://www.japaneseswordindex.com/nihonto.htm