How to convert a drawing from inches to centimeters (or vice versa)?

First of all - an AutoCAD drawing in the modelspace is unitless. A drawing unit can be anything you wish. A square of 10 x 10 units can be 10mm, 10m, 10", 10 feet, whatever. The real physical units are required only when you want to represent the drawing model for the real world - for dimensioning, plotting (plot scale), paperspace and annotation functions.
But if you really want to scale your drawing from inches to millimeters, centimeters, meters - or vice versa - e.g. from centimeters to inches, you can use the SCALE command.

When going e.g. from inches to centimeters, 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;0.3937;