I a response to comments on this blog post, someone named "James Rice" (who is a Microsoft employee) said:

Yes, we have eliminated the upgrade edition of Visual Studio Professional from our Visual Studio 2012 offerings in order to simplify the product line.  We lowered the estimated retail price for Visual Studio Professional from $799 to $499 to make sure this wouldn't have an impact on customers who have historically purchased the upgrade (which was priced at $549). And of course if you need any platforms for your development projects, the best option is to buy an MSDN subscription which provides you with new versions of Visual Studio and all the included Microsoft platforms as these are released.