If you're only using parts of the original work, as might be the case if large portions of the addon required rewriting from scratch, then you're creating a derivative work. Techincally, a modified version where you only changed 1 character on 1 line of code is also a derivative work. You could argue that the new portions are covered under a new copyright, but any portions reused from the original work are still copyrighted to the original author, and if you don't have permission to use them, then your whole work is considered infringing.
Parodies are slightly different. Parodies -- in the US -- are usually allowed under fair use, being considered a form of commentary or criticism. Updating an addon to work in a newer version of WoW than it was written for isn't a comment or criticism on the artistic or social merit of the addon, so it can't be called a parody. Updating an addon would be more like taking Shakespeare and updating all of the dialog to Modern English. That's not a very good techincal example since Shakespeare's works have long since passed into the public domain, but I think it's a good conceptual example.
It's also important to note that the legal status of parodies varies greatly from one country to another; while they're generally allowed in the US, they may be considered direct infringements of copyright law in another country whose concept of copyright encompasses things like moral rights that US copyright law doesn't address at all. The basic tenets of copyright -- eg. Person A created it, so Person B can't use it in their work without permission -- don't vary much between countries, and are covered by several major international treaties that normalize how works copyrighted in one country are respected in others.
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