I was wondering if Capitalism and Communism have always been systems whose adherents are always at war? When was the first instance of a conflict between the two?
As an add-on, what was the earliest switch between systems and what was the most peaceful switch between systems?
It's important to note that communist theory was born out of the first industrial revolution. The growth of factories and the ways in which labor was used by those in power gave rise to the ideas that Marx, Engels, and others put to paper. Petite-bourgeois capitalism -- a trades worker earning a living by their own work, or a small concern with a couple of employees -- was not at all repellant to Marx. It was the systematic extraction of maximum labor for minimum wages that he believed would spark revolution.
When people became a commodity, it marked a major shift in both economic and political thought.
BOMK, there has never been a non-violent switch to an ostensibly communist regime. There have been many small, community-scale attempts at pseudo-communist government and numerous semi-utopian societies, but as far as nation-states go, the switch has been by force.
It's also important to note that while many use "communist" as simple shorthand for the political regimes of (for example,) the USSR, China, and Cuba, none of these states exist in anything close to actual communism. It varies by state, but each of these is (or was) in the process of revolution -- a series of steps that would take a society from a class-based, exploitative capitalism to a classless, cooperative (communal) society. No state has actually achieved communism.
EDIT: Words
Communism is, by its very definition, a reaction to intrinsic shortcomings with capitalism as perceived by Marx and Engels. It was a purported system to right the wrongs of capitalism, so antagonism was inherent.
As for your question regarding switches between "systems", I find this hard to answer. Communism isn't just an economic system (opposed to capitalism) but also involves a political system. It's hard to find examples of situations in which one aspect changed without the other changing as well - and political upheaval usually brings along violence. One could consider the demise of the USSR, in which both the economical and political doctrine transitioned from communist single-party state to a (more or less) democratic government in a rather peaceful fashion. If we limit our scope to the economical system, one might consider China's transition to a semi-capitalist system. The other way round, I can't think of any examples of any such transitions taking place peacefully.