I think the WordPerfect Office X6 Home and Student Edition has the same features as the Standard Edition, except for not being able to import PDF files (it can create them). No need to "upgrade" since it's a stand alone version, and it can be installed on 3 computers in your home.
Check the Corel site (they sometimes have discounts). You can download a trial version which is identical to the retail boxed version (additional components can be downloaded once you pay and register it). Or just buy the retail (boxed) version from Amazon.com, which might be less expensive.
For what is new since X3: http://wptoolbox.com/tips/New-Diff.html. All WPO programs are full suites, so you cannot just buy WordPerfect.
To migrate your customizations from X3, see "Updating, upgrading, or reinstalling WordPerfect" -