I have a Child Portal on a dnn installation that I need to change to a Parent Portal. Is there a script, tool, or method that will transfer the child portal's registered members from one database to a new one along with the settings and third party models? Or am I going to need to build everything from start. The domain alias is registered and needs to be transferred too.

You probably don't even need to do that.  There is very little difference between a parent portal and a child portal.  Since all of the data is stored in a single DotNetNuke database, you won't need to transfer anything.

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Even if I need to put the child website (portal) on a different dnn installation with a different database?

That is a completely different story! 

Probably the best way to do this is to make a complete backup of the site and database.  Install them somewhere to work on them, and delete all of the other portals.  You may have to deal with subscribers separately.

Leave it to me to need something no one else has needed to do. I have 4 portals on this installation: 0,1,2, and 3. Portal number 3 needs to have room for other child portal expansions, and needs to be a parent. You think I should copy the site and db to say, a folder on my dev machine, delete the other portals? Will that make this a (the) parent portal by "default" so to speak? I found a product on Snowcovered.com that will extract the subscribers to an xml file to be restored to a separate db. I am just a little nervous about this... sorry if I am a bit air-headed about it all.

I have recently had to do this for about 10 child portals and what Joe's saying works fine. Copy the files and database for each child portal, renaming each database and updating the web.config. You can go to the Host Setting menu and update the Host Portal to the intended child portal. I would also check the PortalAlias table and remove all entries for each of the deleted child portals if they aren't removed when deleting the other portals using the Host Portals menu.

On a seperate note, if you happen to be hosting at PowerDNN they have all of this scripted in their own tool and it does everything for you in a minute or two.

Thanks boys. I was able to make the transition fairly smoothly. All is well.