Here is what I would do
Basically you want to follow the directions from the wordpress site. Here is what I would do.
1) do an export of the database
2) backup the wp-contents folder and the file wp-config
3) get a copy of the wordpress zip file for your install (2.7.1)
With this, you can recreate your site
You may want to load an index.html file that says 'site down for maintenance' while you do the rest of this.
1) get onto your site and deactivate all plugins
2) download wordpress 2.9.2 and unzip it
3) on your server - delete everything from your root wordpress folder except for any .htaccess file, the wp-config.php and the wp-contents folder (and index.html if you loaded one)
4) upload the contents of the unzipped wordpress folder, to your server
5) update your site by usng 'http://example.com/wordpress/wp-admin/upgrade.php' - make sure to put in your site name.
6) reactivate your plugins - updatating any that need it
ok, you should now be upgraded to WP 2.9.2 but you are still at 3.3.2 - go check things out (get rid of the index.html if you loaded it - better yet, just rename it)
Now to upload Atahualpa
1) down load version 3.4.6 from http://wordpress.bytesforall.com/?p=87
2) unzip it - it will create a folder atahualpa346
3) upload this folder to the wp-content/themes folder
4) go to the theme picker and activate the theme
That's all, you can now switch back and forth between themes versions. If you had any images stored in the 3.3.2 Atahualpa folder, you will have to copy them into the new atahualpa346 folder to see them
Now, go do another backup!
You may need to copy