Thanks for asking! Etymology is always tricky and uncertain, but the most common resource for the history of English language usage is still the Oxford English Dictionary. I don't have a print copy close to hand, but they have a handy lookup at OED.com. The following early examples are listed for "reversal of fortune" and the related, older "reverse of fortune."
1842 169/2 It is therefore a great rarity to see an elderly person or, if an aged one be met with, it will be found that he or she has a tale to tell of sudden reversal of fortune, and dread of the workhouse.
1905 20 554 This reversal of fate should, in order to create unexpectedness and, therefore, interest, be brought about by one who is related to the principal character by ties of blood or friendship.
1948 W. H. Auden Guilty Vicarage in 196 406/2 As in the Aristotelian description of tragedy, there is..also peripeteia, in this case not a reversal of fortune but a double reversal from apparent guilt to innocence and from apparent innocence to guilt.
1656 J. Fowler 216 But before his Commission was issued, a reverse of fortune, of a Commander rendered him a Captive.
1688 in J. Barker ii. 61 Learn from me the sad reverse of Fate, 'Tis better to be innocent than great.
1735 J. Hughes tr. Fontenelle (ed. 3) i. vii. 27 A terrible Reverse of Fortune! And pray who perform'd this noble Exploit?
So, 1656 was the earliest example of the phrase that the editors of the OED could identify. Forgive me for taking a second to stump for one of my favorite books, Simon Winchester's The Professor and the Madman
, and thanks for using AskOPL!