It is true that any group of interacting parts can be considered a 'system'. In this sense, human society is a system; however, like any system, the whole cannot be changed without changing the individual parts, and changing the individual parts changes the system as a whole.
They system is only conceptually real; the actual components are individual people. In this case, the individual components that have conception of a system behave differently because of that conception.
The point is that the system isn't fixed; it is the product of whatever the individuals are doing. The system can't be changed without affecting individuals. The problem isn't a bad system; it's problem individuals, shielded by popular conception of systems. For example, where a problem individual is defend by the legal system, they can be defeated by convincing the populace to ignore the law. This is an example of civil disobedience, which can be made manifest, for example, in the instance of a popular revolution.
Actually changing individuals, even those on top, doesn't tend to do much. They are pressured by the rest of the system (peers, internal and external laws, logistics, economy, etc.) to behave accordingly, or be expelled. A good, honest cop can't do much, for example, if the rest of the force is corrupt and the laws shield them from repercussions - he will soon become an outcast, unless he shows some solidarity with his peers. Even changing people on the top more often than not fails - history knows many potential great reformers, who had to combat their upper classes to make any kind of change.
Convincing people is much better idea, but still it's impossible if you treat them as individual people, detached from the system. The reason is simple - the ideas spread inside the system are usually those most favorable to its stability. Mass media, for example, tend to marginalize parties and ideologies which are not the part of the mainstream, because their owners want to be taken seriously by their readers and (more importantly) advertisers. This, of course, makes contacting the society even more difficult for these groups, which isolates them even more. Soon you can't even identify with these ideologies without being labeled as a ridiculous utopist or dangerous madman, even before you manage to say anything. That's why, for example, anarchist demonstrations doesn't tend to change much - these people has been already labeled by most of the society as wackos who can't and shouldn't be treated seriously. Even if they are mentioned anywhere in media, it's usually with contempt or amusement.
If you want to convince anyone, you need to know how to do this effectively. Which ideas actually help you and which actually hurt your cause and should be avoided. Which parts of the society really are necessary and which kinda emerged because of some strange assumptions which have little basis in reality. You can't do that only by looking at the individual parts.