I'm not a Corel employee, but here are some of my thoughts.
1. WPOX6 is -- like earlier versions -- backwardly compatible with document files created since WPWin 6.x..
2. While you can install multiple versions of WP on the same computer (I recommend installing them in numerical order; e.g., WPX3 then WPX6), you should only need to install WPX6.
Note: Only WPX5 (fully patched) is certified to work on Windows 7, and WPX6 (with Service Pack 1) is certified to work under Windows 8. Earlier versions may work okay on these operating systems, but there is no guarantee.
Note: If you bought a Corel retail boxed WPX6 from a retail store or from a legitimate dealer on the Web, rather than as a direct download from the Corel site, you should make sure it is up to date with a service pack ("patch") as soon as you install it. See the Corel main site under the Support tab for service packs.
3. An upgrade version is the same as the regular version -- but purchased at a discounted price.
4. The WPX6 Home and Student edition is less expensive than an upgrade and can be installed on up to 3 computers in your home. (There is no upgrade version of the H&S due to the lower price.) It is very similar to the Standrad edition except that it cannot import/convert PDF files (it can create them). See a comparison matrix here: http://www.corel.com/static/product_content/wordperfect/x6/WPOX6_Comparison_Matrix.pdf
5. For more on these topics: See "Updating, upgrading, or reinstalling WordPerfect" -
6. I have no experience with Windows 8, so if you need help with WP running on it -- or you have questions about licensing Standard or Professional editons on multiple installations -- I suggest you start a new thread rather than reply to this one.
UPDATE (11/6/13): Coincidentally, here's a recent thread on WordPerfect Universe that quotes the Corel license agreement section about installing WP (presumably editions other than the H&S edition) on more than one computer: http://www.wpuniverse.com/vb/showthread.php?35721-Installing-WP6-on-Second-PC