SOSTX

Different process.

1. Install new 1.5.x - with sample data in a different directory (same server)
2. Delete all articles, Cat & Sections, menu Items in the menus (empty trash for all items)
3. Install Migrator
4. Config migrator to point to the old DB
5. Migrate
6. Upload new template
7. Done! (well, almost anyway).

This component to  migrate users, sections, categories, contents, menues and modules and 3rd party extensions of an external database.

Supported extensions: Artio JoomSEF, Community Builder, Docman, FacileForms, Fireboard, JomComment, Kunena, Letterman, Virtuemart

MTWMigrator

http://www.matware.com.ar/downloads/mtwmigrator/

Tutorial

http://joomlaforbeginners.com/joomla-1.5-video-tutorials/migrating-your-joomla-1.0-content-to-joomla-1.5.html