In that example, the original ancestor was more complex as it had the genetic information from which the mutation would arise. The new species would not be able to change back, so it's less complex.
Wow, so much wrongness in just two sentences.
Let's take a step back and think about what information is. First and foremost it is characterised by unpredictability. If you can infer the next bit of data from what has come before, then it is not telling you anything new, i.e. its information content is zero. If you have a good idea from previous data but it are not 100% sure, then it will contain some information (but less than the amount of data). If you have no idea at all what the next bit of data is, then it is all entirely new information. There is formal mathematics behind this but I hope you get the idea.
So, if I have a counter, that just produces the numbers 1, 2, 3, 4,... then it is not generating any information. If I ask the question: what will be the 2467th number be? The answer is obvious.
Now, what if I have a true random number generator? In this case every number it generates is new information because it is totally unpredictable. Before it generates 2467th number, I will have no idea of the answer to my question about its value. When it generates it, I'll know the answer, so I will have new information. Now, it's not very useful information and, unless its generating lottery numbers or something, nobody is likely to care. That doesn't change the fact that it's information.
Mutations, while not strictly random (some types are more statistically likely than others), are random with respect to survival and (apart from the general statistics) are unpredictable. We have something like a random number generator. Mutations generate new information. Almost all of it is about as useful as the output of our random number generator but the point is that it is new information that cannot be predicted from the previous generation's genomes. So no, the ancestor did not contain the information for the mutation.
So mutations are constantly generating new (largely useless) information. Each human has about 60 to 100 mutations (last time I looked it up - there may be a better estimation now but the exact number is irrelevant), but most of them do nothing of any significance at all.
The point with evolution is that all this new information is fed into the filter of natural selection. Just like you can generate any sound you want from (random) white noise, using the right filter, natural selection amplifies the information that aids survival and reproduction in the context of the environment of the population and filters out any that is harmful. There is nothing really mysterious or difficult to understand about it. If some mutation (new information) is actually useful for survival and reproduction, then those individuals that have it will survive and reproduce more than those without it, and hence it will spread through the population. Conversely, any mutations that are detrimental will quickly die out. Hence new information enters the genome based (essentially) on the environment, because it is that that provides the filter on the constant 'white noise' of mutations.
As for not being able to go back, well it depends. On one level, if a mutation changes a single base from (say) A to G, then it can obviously be changed back by another mutation from G to A. Other mutations are unlikely to be exactly reversed (say a duplication). Anyway, it's unlikely for a mutation to be reversed if it's advantageous in the environment and hence selected. That doesn't mean that, for example, if the environment changes, then the population is any less able to adapt, because it's constantly being fed by the new information from mutations.
Hence, you are simply wrong in assuming that the ancestor is necessarily more complex. However you may define complexity, it can go up or down in evolution. For example, trichromatic vision was an increase in complexity that resulted from the duplication and subsequent mutation of a part of the genome:
The Evolution of Trichromatic Color Vision by Opsin Gene Duplication in New World and Old World Primates.