What does it mean to evolve? An organism becomes more adapted to an environment by developing complexities or becoming the lucky one who didn't. In no circumstance, however, does an organism evolve and not become more complex. The more complex organism might go extinct. The already less complex organism might survive until it too becomes more complex in some other form, but it will not evolve less complexity.
It is often the case that an organism benefits from having not evolved in some aspect. Because evolution means adaptation, the organism is making a gamble that the environment or situation to which it adapted will remain and prosper. All of that complexity helps it thrive when the situations favors that advantage. Take, for instance, a long necked dinosaur. When there are plenty of large plants around it is a good idea to become larger. You have plenty of food and can afford to use more energy requiring even more food. The same principle goes along with the carnivores. Larger prey means they can also grow larger and expect to find ample amounts of the large plant eating species to feed them. The trouble is that now each of these more complex dinosaurs rely on a whole chain of things to not go extinct.
In the first case, this fails and reverts back to more basic dinosaurs. We find that T-Rex comes along because earlier larger species like Allosaurus failed. Now, we get armored dinosaurs. They get large and it makes sense to get bigger again with larger teeth to crush bone and armor. Again, failure. At no point are these organisms getting a second chance to start getting less complex. Instead, creatures like Raptors are becoming more complex in other ways. They are getting smaller with better vision, feathers, more advanced brains, complex respiratory systems, stronger hollow bones, all of which are based on past changes to complexity. They continue to evolve because of the luckier or better choice in what features became more complex. In the end, we get birds; rather complex creatures in every respect.
In no case do we see creatures over time that have had success and not developed some added complexity. In few cases, the creatures have simple ignored almost any evolution at all. This may be the wiser choice ultimately, but it is not always possible. We see during the Cambrian period that becoming more complex was the only choice for many organism. The ones that did not are clearly present in our fossil records for extinct creatures; thus, they did not simple revert to something less complex. The ones that did keep pushing we can look at around us still with their added complexity as all current land and sea creatures.
The bacteria, virus, and other organisms that have not evolved as much likely will not make a big step any time soon. They did not choose to become less complex. They just profit the most from their state. This does not, however, mean they are not becoming more complex too. They are just tied to a lower portion of that chain. The broader your adaptation the more likely you will survive. Adapt complexity to one species if almost certain doom. Adapt complexity to all plants, for example, is a fairly good choice. The trouble comes when you have to complete for plants for which one organism has adapted specifically.
Smaller organisms also often fail from complexity while competing. If a virus adapts to profit from humans, much like the dinosaur example, it too bets that we survive. In the end, if the organism loses those core species the entire chain fails. We see that clearly today in our modern science when we view our ecosystem. Those all exist because of the complexities involved in evolution. None of them seem to be working their way back out of that complexity. If so, we wouldn't see much extinction at all. Nature isn't quite that forgiving.
Even if you could evolve to a lesser organism, which sounds absurd to start with, you would need two things. First, you need time; second, you need the raw material. You can get more complex because you already have some adaptation that will help you survive while you become more complex. Usually, that is the adaptation set that you start making more complex or things related to your already successful survival. It does not work quite so well when you are failing and dying quickly to revert back slowly. Just the same, you could only revert back the feature that you already have. It can't become less complex than it started; thus, in reality, there can't be evolution to less complexity. There are only new beginnings and different approaches.