Was Nietzsche the first philosopher to really lay claim to the concept of "transvaluation", by which I mean the re-evaluation of all values?
Who if anyone before him made that such a deliberate and overt goal?
There is a kind of transvaluation of values in Plato's Republic. Ordinary conceptions of justice, temperance, wisdom, courage, and ultimately goodness, are all 'vulgar' distortions of the real nature of these values as embodied in the Forms. Only those who have undergone the education of the philosopher, outlined in Rep. VII, apprehend what justice and the rest really are. The discrepancy between what the vulgar think and the philosophers know about the different values is so radical that 'transvaluation' is not too strong a term to express the philosopher's new perspective, rooted in reality rather than in the vaguenesss and imprecision of everyday belief.
Plato, Republic, tr. C Rowe, ISBN 10: 0141442433 / ISBN 13: 9780141442433 Published by Penguin Classics, 2012.
No, he certainly was not the first to attempt such a thing (at least if we follow Nietzsche's historical analysis of previous transvaluations) nor the first to think about it, but he may be the first to have thought about it and also attempted it (i.e. the first self-conscious attempt) — and he did, I believe, coin the verb umwerten to describe the process.
Arguably classical Greek ethics was the first attempt at such a thing; that is certainly where Nietzsche got the idea, from his analysis of Socrates and the other classical Greek moral philosophers. But these Greeks didn't think of themselves as transforming a coherent ethical system so much as making an inconsistent (and therefore only quasi-ethical) ethical system, consistent and rational. Once these philosophers became culturally dominant, they didn't really argue that much with each other about the content of ethics (what sorts of duties an ethically person has), but about how to understand ethics and how to become a virtuous person.
(Likewise later when the Christians attacked the moral corruption of pagan culture they were, in Nietzsche's sense, transforming it, but again not self-consciously; they thought of themselves as in most senses more loyal to the true meaning of pagan moral philosophy than the pagans themselves, and the apologetic literature of the early Church constantly points out that ordinary low-born Christians have more virtue than philosophers who devote their life to self-perfection.)
The first people to self-consciously devote themselves to a "transformation" of ethics that wasn't just seen as a perfection or purification of the existing society's (quasi-)ethics were the neo-Romans like Machiavelli who not only admired classical culture, but identified a clear distinction between the virtues produced by classical pagan states and the virtues of Christianity, and preferred (and sought a return to) the latter. This Renaissance episode may not strictly reach the standard of "transvaluation" Nietzsche set for his Overman — the creation of new values, not the resuscitation of values out of museum exhibits (what we would now call LARPing) — but Nietzsche definitely identified it as a predecessor to his own attempt to overcome Christian values. Unsurprisingly, this interpretation of the Renaissance fits well with the then-pioneering studies of the historian Jakob Burckhardt, Nietzsche's colleague.
Examples of philosophical reflection on moral inversion can be found from Montaigne on. Montaigne considered whether cannibalism and torture might not be noble; Grotius similarly set up a hypothetical immoral morality (only for the purposes of rejecting it); Kant considered, not a world where all the moral duties were inverted, but one where the duties were normal and the people were all secretly amoral monsters; Hegel coined the term verkehrte Welt.
But all of these figures ultimately had fairly straitlaced, conventional theories of our ethical duties; the only figure who comes to mind who both was highly unconventional ethically and recognized himself as such was Sade, who only barely qualifies as a philosopher. Between Hegel and Nietzsche there were a number of thinkers with unconventional ethical views (Stirner, Marx, maybe Feuerbach) but they generally thought of themselves as trying to ignore morality rather than fashion a new one.
I guess Rene Descartes did earlier, as his famous allegory of the basket of fruit.
Our mind is like baskets of apples. If one apple is spoiled, then it will make all other apples spoil. To keep all apples good, we have to test all apples before putting them in the basket. The same way, all thoughts, valuation, or its truthfulness we need to test before we adapt them to keep our mind good and not get spoiled with bad thoughts.
He called it rationalism instead of trans-valuation but I do look both concept almost same.