[Mailman-Users] Upgrading and using with multiple domains

[Jon 1234]

I currently have Mailman 2.1.14 installed in the website directory of a.com
I would like to:
1. Upgrade it to 2.1.18-1.2. Have it outside the website directory.3. Use it not only for a.com but also b.com (I will not need to have the same list names for both domains).
If I were to install from scratch outside the websites' directories, I imagine it would be simple enough to get it to work for b.com. 
But what would I need to do to get the a.com mailing list to start using the new Mailman installation? (Would I just be able to move certain files across to reproduce the archive, etc?)
I am sorry for asking such a basic question, and would be happy to be directed to the right FAQs on the subject(s).
Best wishes and thanks
Jon