I have seen a post, which shows how to install W800i firmware on K750i from SE official website.
While upgrading firmware, you need to run given exe file first. Then the phone is detected as W800i and its firmware is installed. Since both W800i and K750i are same phones with different outer case, this won't make any difference to phone functionality, it becomes a complete W800i, without any quality loss.
If you want to revert back to K750 firmware, just don't run the exe and phone is detected as K750, and its firmware is installed.
I have not tested this, try at your own risk.
K750i to W800i Via SEUS and DCU60