CAD tip # 7103:

But if you really want to scale your drawing from inches to milimeters, centimeters, meters - or vice versa - e.g. from centimeters to inches (i.e. between metric and imperial units), you can use the SCALE command.

When going e.g. from inches to centimers, you have to scale up your drawing 2.54x. Run the SCALE command, select All objects, specify *0,0 (global) as the reference point and enter 2.54 as the scale factor.

You can prepare a menu macro button to perform this conversion operation automatically (menu macros use ";" instead of Enter) - some examples:

[Inches->CM]^C^C_SCALE;_All;;*0,0;2.54;
[Inches->MM]^C^C_SCALE;_All;;*0,0;25.4;
[CM->Inches]^C^C_SCALE;_All;;*0,0;0.393701;

To perform a formal change of units without changing geometry, use the command UNITS.

To automatically convert the scale/units of existing objects and blocks to insert, use the command -DWGUNITS: