This has to do much with the hardware and software. You can put any firmware, but that can kill your phone because it happens to modify the GDFS. For example, you can lose the signal in your K800 if you flash it to W850, so you will have a brick used as phone.

There are some models that can be converted into others (only those established, do not try to make up a new conversion).

For K810, you only need the firmware files (Main + FS + CDA) and put them into your K800. Then, you have to add the light effects of your K800 into it because it loses some lights in your keypad when you do this.

Anyway, I suggest you to follow the existing conversions, DO NOT try to make new conversions.