If you keep the blade from getting hot enough to draw the temper while you work, you won't have to re-harden. That's typically less than 300-400 degrees F, but generally keeping the blade cool enough to handle with bare hands, or so that water will stay on the surface without evaporating instantly is safe. Remember the thin edge can get a lot hotter than the rest of the blade if you are careless. That means wet grinding, frequent dunking if you use a powered sander or grinder, or lots of hand sanding. If you are "lucky" your blade might be soft enough to cut with a good quality hacksaw blade. More likely though you will need to use a carbide grit hacksaw blade or two. Be patient, take your time, and figure out some way to hold the blade and protect yourself form the sharp edge of the blank. Look at the various jigs and clamps some of the makers here use.
An easy (but somewhat risky if you have no experience with the tool) way to cut the blade will be with an angle grinder with a cutting wheel. You need to protect the rest of the blade from the heat of the cut though. You can buy "heat stop" paste, and I have read that plunging the blade through a raw potato will also stop heat...never tried it though. Anyway, clamp the blade firmly somehow, protect the part of the blade you want to keep from heat and cut the blade off 1/8' to 1/4" "long". Grind off the part that turned color next to the cut line
