a re grind is pretty simple, going from 1/4" to 1/8" sounds pretty tough, if you only regrind only the blade the handle will stay thicker and it will look weird (really) if you want to reduce the whole knife to 1/8'', blade and tang, you'll need to take it to a surfaçe grinder to do it correctly and accurately, keeping the edge centered, then regrind the blade because after tha tthe grind will look like a scandi .... assuming the surface grinder didnt ruin the HT. i dont see someone taking it freehand to a belt sander to reduce the whole stock keeping the edge centered.
my advice is buy another knife or do a less dramatic regrind. 1/4" with a thin grind, close to zero edge flat grind will cut very well for most tasks.