I have been using the" Add Module: Existing" command to place a module that is required  throughout our site.  The module is in use on about 40% of our site, therefore using the "add to all pages" command would have been impractical. 

I am now in a position where I need to make a change to the content of the module.  Although when I make a change it is not propagating throughout the site.  I was under the impression that so long as you utilized the "Add Module: Existing" that a change to the content on any of a given module would change all of the copies of that module throughout the site.

Is there a specific way to edit module content to enable this functionality? If not, what is the method to create modules to enable the "edit once/copy to all" functionality?

Thanks in advance for your help.

--mvr

And just to clarify that should have read "... make sure you do NOT tick the Copy Module box before clicking Add Module." Ticking the Copy Module box adds an independent copy (changing the content only affects that one module) whereas not ticking the box adds a relative copy (changing the content of any relative copy changes the content of all copies). I learnt the hard way as well!

That is correct behaviour as regards the settings.

You might, for example, have a site that uses different skin colours for different sections (as I do) and need to match the module colour without affecting other module copies on pages with different colours.

AFAIK there is no way around this.