What exactly is your source for that? The coatings on an 800K disk are thicker and less coercive. Meaning, the DD disks are have more particles but less magnetism. This is used with a stronger write signal, but fewer bits encoded. In HD media, there are fewer particles, but more magnetized to accommodate the weaker write heads and more more bits encoded. The purpose of this is to cut down on track interference. Therefore, in putting an HD disk in a DD drive, you are combining greater magnetism with a stronger magnet writing the information and resulting in a greater risk of oversaturation and track bleed, despite writing a lower density of information. Likewise on the DD disks, you are using weaker media with a weaker HD drive magnet resulting in poor recording of much more information and increasing the likelyhood of bleed and readability.
Just DON'T DO IT, except for experimentation. Also, the likelihood of buying an unformatted HD disk seems almost improbable to me today. You cannot overwrite a pre-formatted HD disk and expect it to ever function in any way reliably. DD disks are still widely available and inexpensive. Check eBay, lots go up for sale there all the time.