Very interesting premise. If I'm not mistaken, the Mayans believed in different eras, each marked by one of the four elements. Each era was ruled by an element and would end by that element. Accordingly, we're in the era of Fire, and this age will end by fire. That could be something to play around with.
As for transformation...if I found out that I would inevitably transform into something inhuman, I'd be frightened and depressed at first. Even though most people aren't always entirely satisfied with their appearance, to change into something that's altogether inhuman is a terrifying proposition. Even if the new shape is more powerful and capable of magic, I think plenty of people would be filled with dread at becoming something alien. I'd expect to see a high increase in suicides.
People who know whats coming will probably try and prepare themselves for it. I could imagine a lot of pop psychology and self-help books hitting the market. "Embracing the Change" or something like that.
Something very interesting to consider is the shift of power. What happens when Terrans outnumber the normal humans? I think up till that point there will be attempts at genocide, Terran rights groups, and the kind of racial tension prevalent in America during the Civil Rights movement, only on a much more massive scale.
I'd expect certain governemnts to crumble, but bigger, more stable ones, like the global superpowers would probably remain intact. I assume that most people would maintain the same interests, and ideals even if their physical aspect changes. I would expect to see a lot more cases of insanity. Some might be permanent, others temporary. Of course, this is also dependant on the totem guide. How influencial are they? Will they try and guide the newly-changed Terran in ways other than magic? Could they influence their social and philosophical mindset as well?
One thing that I'd recommend against is making Terrans "better" than normal people in all ways. If they're naturally wiser, more moral, and better natured than humans, as well as being magically gifted and more powerful, it might cause readers to roll their eyes and think "Mary Sue...on a global level..." (If you've ever read any of the Eragon novels, you can see how Paolini treats the Elfin race as an entire species of Mary Sue characters.) Not sure if that was a possibility but wanted to warn against it, just in case.
I hope this was helpful and the kind of input you're looking for.
