In Sharepoint 2010, is there any way to change the parent (or base) type of a Content Type after the relationship has been established?
Given the way that ContentTypeIDs are created, I would be very surprised if there was anything in the API that let you do this. However, since the Content Type is not in use yet, you should probably simply delete the current one and then recreate it based on the desired base type.
A Best Practice is to create your own Content Type derived from one of the SharePoint types and then use that as the base content type for all other custom content types you create. This lets you optionally add new columns to the Base Type without having to break all of your custom content types.