Ahm - i had the odd thing happen to me: one model which i started in “architectural mm” decided to go rogue and change itself to “architectural meters” …well, let them have their fun while i’m away but…when i go to settings yadiyaada and try to change it back to mm it…refuses that? any clues?
sincerely m.
“Architectural” is meant for Myanmarians, Liberians and Usonians. It is feet and inches only. Choose “Decimal” first.
As @Anssi suggests, the real puzzle here is why you thought you were using “architectural mm” in the first place. There is no such setting in SketchUp. I’m not even clear what “architectural” would imply if attached to mm or meters.
Edit: on my Mac, I don’t even see those choices unless I click the “more templates” button!
You go to Model Info>Units. There is no Settings yadiyaada window in SketchUp.
Aha! Since I almost never use that startup screen, I hadn’t ever noticed that. All the metric “Architectural” choices seem odd to me. In what ways other than units do they differ? Do they differ at all from a model in which you simply set meters e.g. as the units?
Edit: on my Mac those choices don’t even show unless I click the “more templates” button. That’s why I haven’t seen them before!
AFAIK, no.
The sky in Simple is blue and is grey in Architectural.
A sorry comment on architects 
But all the Architecturals are the same. Architects are supposed to like grey.
It is customary to put in CDs the definition “colour by architect”. In my case it might vary from red to white-grey.
The ‘Architectural Centimeters’ template is fine for a light theme, also the name.
yo, aside from all the maynmaresque fun to be had and architechtural excurses…let me explain, a bit more:
There is a thing called “settings” in sketchup, at least if one would translate “voreinstellungen” as settings (which is a common thing todo, according to merriam websters…)
and if i change the so called “yadiyada” (which in fact would be called “template” i guess…)
from “architectural meter” to “architectural mm” …nothing happens. Or is that not the way to change things?
curious and curiuoser…
m.
If you want to change the units in an existing model file, do that in Model Info>Units or whatever that translates to.
Choosing the template in Preferences just sets the Default template for the next time you start a new file. It does not change the currently open file. If you want to choose a different template do that from File>New from Template. It won’t make a change to your current file. If you’ve started a model and want to change the units, do that in Model Info>Units.
Architectural Millimeters:
Architectural Meters.
ah. thanks. that what i wanted to know. is there…any reason why the units would…change all of a sudden from working on it, saving it, opening it up again…and hopsa - you’re now in a different scale??
I’ve never seen that happen in a SketchUp file. I’ve only been using SketchUp since 2003 though so…
theres not like a superweird keycombination that does that…? cause it happened to me twice now. but it might have to do with my weird ass computer ^^. might very well be possessed.
No. Unless you have an extension installed that changes units with keyboard shortcuts.
Maybe you just weren’t holding your mouth right. 
