There is really only one difference between 400K and 800K disks. Disk manufacturers would test only the first side of a 400K disk for errors. The second side, used only when the disk is formatted as an 800K disk, was good over 95% of the time. In the olden days, we used to buy 400K disks (they cost less than 800K disks) and format them as two-sided, almost always getting ten perfectly good 800K disks out of the box. Eventually, the single sided disks were pulled from the market and those who wanted 400K disks would only format one side of the 800K disk. 5.25" disks were sold the same way.

In other words, unless you absolutely must have the disk box for your collection, don't bother with this. 800K disks will cost less and provide the same performance, perhaps with a slight increase in reliability.