Yes, you can change the DEFAULT SCALE LIST in an existing drawing.

Born and raised in the States, and having worked for decades as a carpenter, I am quite familiar with the Imperial system of measurement. I remember hearing in elementary school in the fifties that we would soon be switching to Metric, yeah right. After almost 20 years living on the other side of the pond, and having become fluent in Metric units, I now find having to deal with fractional conversions beyond tedious. Occasionally one of my coworkers will supply me with a 3D Model for detailing, which has erroneously been created on an Imperial template, it makes me nuts. I routinely vet incoming models using the -dwgunits command, inform the miscreant in question, and change the units to millimeters. But something that has always annoyed me, was that there appeared to be no way to then change the DEFAULT SCALE LIST (on the User Preferences tab in OPTIONS) values from Imperial to Metric. Scales would continue to be displayed Imperiously, like 3/8"=1'-0", requiring me to think way harder than I ever need to working in Metric units.

I was quite pleasantly surprised to discover that there is a way to do it! Resetting the System Variable MEASUREMENT, which controls Hatch and Linetypes used by the system, as shown in the image will flick the switch. The Default Scale Lists follow suit.

In order to ensure that the scales, area and volume units reported in PROPERTIES comply with your new scalelist units,you may need to use the SCALELISTEDIT command, and RESET option (thanks to the Ber for the name of the command, and to rkent for another tact).

As rkent pointed out, the same dialog box can be reached through the VIEWPORT SCALE list, with the CUSTOM and RESET options.

And you just found this out? You? The master of system variables? I don't believe it for a minute. Is today April 1st? It's got to be.

Sorry, I didn't realize it was common knowledge,

felt like I had really stumbled onto something.

I have wanted to be able to do it for as long as I have been receiving this sort of file.

Most of what I DO know about system variables comes from when I found out about SYSVDLG,

and from having spent an hour or two reading through a lot of the variable descriptions.

Lots of them are of absolutely no interest to me, as I would never have need of them.

Had there been any mention of this additional feature of this particular variable, I certainly would have

seen it and been thrilled. Lamentably there is not, and I was not.

I knew that changing it on the user preferences tab wouldn't stick.

Odd that they should make it look in the DEFAULT SCALE LIST dialog box like you can choose whichever

default scale list you want, when in fact the selection option provided there does nothing.

I kind of thought that the -dwgunits command used to do it, seems like it should.

I guess anything is easy once you know how to do it, eh?

I never work with any metric drawings except for helping troubleshoot here and at the Autodesk forums. I knew about MEASUREMENT, but I only knew that it changed the default line and hatch files, I did not know it also reset the scalelist. Thanks for the tip.... I actually could have used that in a topic a couple days ago.