I guess that is the quote that's been given as "If anything is certain, it is that I myself am not a Marxist," and in other variants. So far as I can make out it referred to tactical disputes among French Communists. I'm not sure I understand it correctly, but it seems like one group of French Marxists was very radical and dismissed reformist planks in the platform. Marx felt those promises of reform were necessary to build the movement.
Marx wasn't thinking reform of the existing system was enough, but he recognized that more workers were attracted by promises of reform than by violent revolution. Or maybe I'm totally wrong about all that. Marx's son-in-law was one of the French Marxists he attacked, so maybe it was a family thing. In any case it was a throwaway line that proved useful in debates among Marxists later, not a thoroughgoing repudiation of his earlier thinking or of what his followers were doing.
I guess there was a young Marx and an old Marx -- or a bunch of different Marxes spread across his lifetime. It may be that the big change came when Engels and others tried to systematize "Marxism" as a "science." They could have said "these are some ideas our pal had," but instead they presented Marx as a proven system that could predict the future. Marx probably did think he could predict the future, but if he'd lived longer he would probably have gone on to contradict things he'd already said.
From my understanding KM thought that the change of system would not be a violent overthrow though he recognized that the bourgeois would not surrender power without a struggle and was in control of the state instruments of oppression: the police and the military, I think he believed that change would come because the internal contradictions of capitalism would produce the next stage of economic development.
I always found it kind of ironic that his magnum opus, Kapital, was based on the research he did in the British Library where he utilized government reports for his data.