Certainly, Marx and Engels believed that the proletariat of the time period had the power to make a social change. Marx and Engels believed that the conditions were right in industrialist nations like England in which the proletariat had amassed in numbers and strength to force the issue of change. The Revolutions of 1848 might have hashed this idea out a bit. Certainly, there was some level of thought that indicated this being the case. It lies central to Marx's thought that the proletariat in industrialist settings would be able to initiate a form of change.
Whether or not this change is realized might be more of a reflection of a potential limitation in the philosophy of Marx and Engels. At some level, the thought that emerges from The Communist Manifesto is that social and material conditions will prompt change. This has to be debated, to an extent. Capitalism demonstrated itself to be pliable enough that the proletariat did not see the need to overthrow the entire structure. Marx and Engels did not see this in their work, displaying capitalism to be rigid and incapable of modification. It is here where I think that the questions raised help to bring out whether the proletariat had the power to create social change, or if they demonstrated willing to do so if capitalism could be modified to include a bit more of their own interests.