You can make a copy of the installation files and db, move it to another installation. If the host name is going to change then you should add the new host name mapping to the site before you copy it. You can run a query in the db to set the IsServerAdminSite flag to true on the site row in mp_Sites if its not already the root site in the previous installation. Then you will be able to delete the other site using site settings. I did this myself long ago, I had both mojoportal.com and joeaudette.com running from the same installation and db but I separated them into different installations and different dbs (both MySql).
However, changing from one db platform to another and migrating content across db platforms is another story. I have no solution for that at this point.
Hope it helps,