There are several upgrade paths, depending on what you’ve already purchased from Omni in the past. (Before the universal update, the iPhone and iPad apps were paid, separate purchases, with only iPad carrying a Pro upgrade version.)
It’s not the easiest upgrade process to understand, but here are two charts from Ken Case (via the Twitter) that will help:
And check out this lovely screenshot from the updated help files. You can now re-arrange your Perspectives on the phone.