|||| Upsherin's first written record in Rabbinic literature is in Sha'ar Hakavanot by Rav Haim Vital. There he writes that his teacher - the Ari Z'L cut his son's hair for the first time during his third year on Lag B' Omer...according to the well know custom.|||
(below is from Seth Mandel)
R. Chaim Vital recounts numerous things that he heard about his rebbe, the Ari, as well as things he himself saw the Ari do, but on the matter of the Meron pilgrimage and the upshirin we find one of the few places where R. Chaim Vital saw fit to put in a cautionary note stating that "the Ari may not have done what he did if he knew all that he knew later." (For another case, see the issue of the putting in tzitzis in two adjacent holes; for a case where R' Vital accepts what he heard without a note, see the issue of haqqofos on Simchas Torah.)
The fact that R' Vital put in the note (which, of course, is ignored in the Eshkol edition of Tammai Haminhagim, even though the note is right after the quote that IS included) shows that R. Chaim Vital knows of no support for the custom in anything else that the Ari said, and it certainly suggests that the Ari never did it again.