What are the differences between an upgrade licence and a normal licence?

I have noticed that there are different types of “Pro” licences that can be purchased for eM Client:
-The standard $49.95 licence (http://www.emclient.com/purchase?lang=en)
-The upgrade $29.95 licence (http://www.emclient.com/upgrade-to-pro)

It’s implied that the upgrade one allows users to upgrade from the free version to pro. However, wouldn’t the standard licence also allow users to do the same ‘upgrade’? The order form names the two licences differently, so they do seem to be different products.

Besides the price, are there any differences between these two licence types?

These are not different types of Pro version - we just decided to offer PRO license for lower price for some FREE users.