If you want to flash your phone from K750 to W800 then use FS & Main of W800. Before downloading also check phone color red/brown because FS and Main are related to color as well.

For flashing use XS++ with DCU60 cable (DCU 60 comes with phone) and use FAR Manager to customize (Note: FAR support only Red color and not brown). If your phone is brown then first convert it to Red. You can browse this forum for guide to convert Brown to Red.

I debranded my K750i by XS++ (to flash main & FS) and FAR Manager to customize. I hope that you will be able to flash your phone successfully.