I already have an installation with multi sites and want to add Google Analytics.

What are the procedures to change the existing IDs?

Perhaps you could have a look in our free Google Analytics module that also allow you managing several domain name with specific google IDs.

See the "free download" menu.
www.jms2win.com/en/component/docman/doc_...tes-google-analytics

Well,
I'm beginning to love JMS and logically Multi Sites Google Analytics

What I don't understand is Why do it as Module instead as Plugin ?
Isn't Plugin a better alternative to the "module based" solution ?

Yes it could be a plugin but I have prefered a module to be able choose the position where to include the code.
The old Google Analytics code was prefered in the bottom of the page (or the debug position).
Now the new google code is prefered in the top of the page or header.

In addition, this give the possibility to include several Google Analytics in the same page with different codes. With a plugin, you can enter the code only once.

So you see that you have several advantages to have the Google Analytics defined as a module instead of a plugin.

Google analytics gives you clear indication of every thing about your domain, you can register your domain their check for the details.hopefully it will allow.

Are there very specific instructions on how to setup multisites Google Analytics? The download page does not give a place to see documentation and also does say if the plugin needs to be installed on all the other sites. Do you we just declare all the multisites GA codes in the master site? There seems to be a lot of instruction missing in order to get it working.