Funny, but I've just read separate posts from two owners of US spec IS300 cars where the outside temperature display changed from fahrenheit to celsius after the installation of a new battery. In one case, the owner took his IS to a Lexus dealer and the dealer could not figure it how to change the temp display it back to fahrenheit.
I'm wondering if there is an HVAC ECU that can be programmed with the scan tool and program card to toggle the outside and inside temperature displays between F and C -- except this is not an option listed on the list of customizable LPS settings I've seen for any Lexus. Or perhaps there is a method of toggling the display between F and C like the method an owner can use to vary the automatic fresh air/recirculated air sensitivity by pressing the AUTO button and then using the driver side TEMP button to vary the sensitivity between -3 and +3 -- at least, thats how it works on the LS. If there is such a method, it is not documented in my LS owners manual or the two volume repair manual set I have.
I noticed after buying my first Lexus almost 20 years ago that the celsius based HVAC display modules in the right had drive European LS was visually identical to my fahrenheit based HVAC display module. Being the sensible company it is, it is doubtful that Toyota would have two separate HVAC control modules - one F and one C - for different markets.
I never thought about inspecting the back of the HVAC module in my 00 LS when I removed it and the radio a few months ago. I didn't notice a switch on the back of the HVAC module but that doesn't mean that there wasn't one or that there isn't a hidden switch. But I'm betting that, if the display can be toggled, its a programmable option of some sort.
Or .. you could think of the conversion between farhenheit and celsius as mental exercise.