On the first and third knives, you can fashion a spacer out of fiber, plastic, hard leather, or similar. Cut it in the same basic shape as the handle-with the center cut out to fit over the tang, then cut a seam in the bottom portion of the spacer. Force it between the guard and the first spacer until it fits in place. When done, it should put enough pressure on the rear of the guard to take up the slack from shrinkage. If it does, then a drop or 2 of Super Glue will hold it in place. Once dry trim/grind/file it to shape.
You can try something similar on the rehandled knife, but it will show. Or, you can heat the blade just until the glue holding the handle breaks loose. Pull the handle off. Clean the blade tang of all old glue, fit the guard tightly, and use lead solder or silver bearing solder to hold the guard in place. A good soldering flux will be necessary. I use Stay Clean flux. Parts to be soldered should be sanded to bright metal and be very clean before soldering. Reglue the handle, and clamp with rubber bands around the guard until the epoxy has cured.