Hi Mark, first go check your settings under Options > General > Unit options, it's this button here:



In the small dialog that comes up, pay special attention to the "Scale on unit system change" checkbox, you want to make sure that is turned on (checked). It is checked by default but maybe you've turned it off at some point? That dialog looks like this:



You can also change the default unit system there which controls which units are used when you initially start up MoI, unless you have set a template file in which case the units in that template file will be used.

Then go to the Unit system dropdown in the main Options dialog, here:



That controls the unit system setting for the current document (as opposed to the startup default units which what the setting on that smaller dialog controls).

Now change that from your current Inches setting to Millimeters instead.

That should change the current units and also scale objects into the new unit system as well when that one checkbox mentioned above is enabled. So for example in if you had a box that was 1 unit in width in inches, when you switch to mm with that checkbox option enabled it will become 25.4 units across after the unit system switch, since 1 inch = 25.4 mm

You'll probably want to leave that "Scale on unit system change" in its default enabled state normally, the only time you'd really want to disable it is if you want to switch unit systems while having everything maintain the exact same numeric coordinates (like a thing 1 unit in size staying 1 unit in size even when the units change), which you normally only want to do if something was done in the wrong units initially and you want to change the units label and nothing else.

Hope this helps! If you're still having problems maybe you can post the model file so I can give it a try over here.

- Michael