Politics offers several opportunities for surprise reversals. You could talk about almost any political candidate, his/her promises during an election, his/her popularity before an election, and then the legacy he/she actually left behind.
If you want to stray from politics, you could also write about popular (or seemingly popular) products and their actual efficacy. I am always in awe of the hype that surrounds new beauty products (especially where celebrities are involved) at the very beginning and then the actual customer response once the product has been on the market a while.
Finally, a more personal idea might be to write about your thoughts on high school (or a certain year of your life) before you lived it and after. I think again about how anticipation of an event can greatly skew the actual results/emotions after the fact.
Another possibility could be the way we have defined civilisation in terms of progress and then have evolved into destruction and excess. A possible angle for this could be diet and nutrition and how people have evolved from starvation, to subsistence and into excess. The wealthier nations now have poorer health (certainly in terms of obesity) than some of their third-world counterparts.
If you want to discuss a very current topic, you could try unions in America. If you wrote an informative essay on what caused labor unions to come into existence and how they originally prevented employer abuse of employees and helped establish equality, you could then add a reversal about how today's unions (in some cases) now hurt the average employee while the leaders of the unions benefit.
Here's one for you -- the topic of oil and the Middle East. We tend to think that we need to be involved in the Middle East because of the fact that we need their oil. This is why we are so concerned with the stability of places like Iraq and Iran and Saudi Arabia.
You could write an essay about this -- talking about all the reasons why we need to spend so much money protecting the Middle East. Then comes your reversal -- we only get 10% of our oil from the Middle East.
A reversal like this would make people rethink their assumptions about our involvement in the Middle East.
In terms of writing an informative essay, I would think you could choose any number of topics that would lend themselves nicely to a reversal of topic.
Generally, I would think that a surprising reversal would have to come about in the discussion of a topic people care deeply about, or the surprise would have to be spectacular, to have the desired effect.
If you were, for example, to write an informative essay (also known as an expository essay) on why abortion should be banned, a surprising reversal might come in the form confronting the issue of a young woman who has been raped, or a youngster who is the victim of incest or sexual abuse.
If a strong opponent of abortion were to read an informative piece on what abortion is and why it is wrong, and then be confronted with the necessity of making an ethical decision involving rape or molestation, I would assume this would serve as a surprising reversal. Two extremes would be necessary, I would expect: surprise (and discomfort) are present whenever we are forced to consider the other side of an argument—to look at the morality of a situation from all viewpoints.
I'm sure you also use topics such as assisted suicide, stem cell research, capital punishment, etc., just to name a few.
I hope this is of some help.