I do not believe the 'idea' of democracy changed because of the American Revolution. I do believe that a commitment to and passion for democracy changed because of the American Revolution. The Revolution made individuals and groups aware that democracy could be achieved somewhat through organized uprising against what was perceived as stifling, tyrannical authorities - akin to what we're witnessing as evidenced by the revolts in Egypt, Libya, and such. The American Revolution's long-lasting effect is the way it inspired generations thereafter to look at their example as a means to achieve their dreams of democracy.
Thomas Jefferson accomplished two primary things when he completed The Declaration of Independence: He brought to the forefront the type of government that the colonist wanted and exposed King George III as a ruler who felt he could continue to harvest and reaps the benefits of the colonies.
The Declaration... created a document which espoused the type of government that the colonists wanted--one based on equality and freedom. In addition, the decisions would be made by ordinary people not an elite set of aristocrats or royalty.
In addition, it became obvious that George ignored the document and felt that he and his armies could control and overpower the newly formed congress. The American Revolution came about because of the democracy that the people wanted. George picked the wrong fight.
I think that the American Revolution provided a new idea of governance. Most people probably did not expect it to last long, but it did. It led to more democratic governments later-usually founded by revolution. Sometimes all it takes is one success for others to follow.
Democracy, as written about by philosophers like Voltaire and Locke and newly embraced by sovereigns like Catherine the Great of Russia, was a new application of an idea in the world, though perhaps not a new idea. The American Revolution changed this new application of democracy by removing it from theory and political debate and making it a reality in the world: a group of people successfully rebelled against perceived tyranny to create (racially limited) liberty and equality. So, in this sense, yes, the American Revolution changed democracy by making a reality out of a newly re-envisioned ideology.
I do not think that the definition of democracy changed. People did not start defining democracy in a different way. However, the revolution helped to make people have different attitudes towards democracy. Before, there was the idea that democracy equaled chaos. This idea gradually disappeared after the revolution and the success of the US.