Where did the concepts of ‘Left’ and ‘Right’ originate, and how? And have those concepts really changed from their original meaning?

Sure it's changed .it goes back to the days of the King Louis XVI of France . During the revolution there was an Estates General. On the left side sat the 3rd estate members and to the right sat members of the nobility or 2nd Estate . In America there was no nobility persay so Right has generally meant traditionalism or conservatism more than monarchialism and left has evolved along almost parallel lines with social democracy .

I can understand the confusion because sometimes the terms have Humpty Dumpty meanings. Fascism as an example has often been misnamed "right winged" when in fact it is socialist in design and construct.

Not to take away from toms answer but to add to it the American twist on a two party system.
Both left and right in America are descendants of Jeffersonian tradition. The Jeffersonian viewpoint is a logical whole, and it is impossible to put one half of it into effect without the other.

Jeffersonian democracy - Wikipedia, the free encyclopedia

Is there any political discourse you can tie into this idea Tom? The idea that conservatives sat on one side and reformers on the other side doesn't carry much weight aside from speculation as to what may be the case.

Was Jeffersonian tradition pulled out of nowhere; I mean was it all something original, or based on some previous notions?

It is based on everything we must not do to stop the Aristocracy from gaining control of a country. Which is what they do, you know, being an Aristocracy and all.

"It's not the votes that count,
it's who counts the votes."
-- Josef Stalin --

As to the origin of the political terms “Right” and “left” I can only find the assertion that the labels originated with – “the days of the King Louis XVI of France,” as you point out Tom. However, I cannot find any Political philosophy expounding on the idea until I get to Hegel.

“Where the Old, or Right, Hegelians focused mostly on the religious aspects of the Absolute in the service of justifying a conservative Prussian state, the Young, or Left, Hegelians were far more inspired by what they saw as the radical implications of Hegel's political thought. For the Left Hegelians, notably Ludwig Feuerbach and Bruno Bauer, the Philosophy of Right provides a blueprint for the reform of European politics, with an emphasis on free markets and participatory democracy. The Left Hegelians also embraced a humanist project: Feuerbach, Bauer, and Strauss, for example, all offered historicist accounts of Christianity, and attempted to show that religion must be understood as a social rather than a divine phenomenon.”

As least in those two respects, the left and right do not appear to have changed.

However; the Left then had reason on its side, while the Right had obscurantist irrationalism. Political ideologies were polarized, with liberalism on the extreme “left,” and conservatism on the extreme “right,” of the ideological spectrum.

What happened that the roles became reversed?