I wonder if there is any chance to remove the 6.3mm plug from the hd800 stock cable without just cut the cable right off?
Nope. Reterminate the cable with a 3.5mm plug if that's what you want or get a 1/4" to 3.5mm adapter.
The oversized stock plug is soldered on and the solder connections sealed with molded on plastic. Too much trouble to disassemble versus just cutting off the plug and soldering on something different.
It does not have to be destruction.
You can cut off the 1/4" plug about one foot or 30 centimeters before the plug, solder on a female 4-pin XLR, and then solder on a male 4-pin XLR to the rest of the cable and you can still use the stock plug as an adapter.