I say, not necessarily.
The j-blogosphere is abuzz about the article that appeared in Makor Rishon which suggests that the pre-existing Lag B'Omer celebration was converted to the yahrzeit of R. Shimon ben Yohai due to a textual error in the copying of R. Hayyim Vital's manuscripts (with "יום שמחת רשב"י" being abbreviated as "יום שמ' רשב"י" and then misunderstood as "יום שמת רשב"י."
(Discussed by My Ober Dicta, who is concerned with whether "cold water" should be thrown on well established customs, Jameel, who translated the article, DovBear & GH, who are reporting the issue, and who knows who else.)
I haven't seen the manuscripts, but I assume that they bear out this claim. After all, either documents showing this exist or they don't. But let's say it is true that such a textual error occurred. Is it then impossible that in the original, "יום שמתת רשב"י" could not have referred to the yahrzeit of, but the celebration of רשב"י?
To this I say: not necessarily. There is a well known phenomenon in language in which words acquire opposite meanings. Whether it's in pop slang or Tanakh (Job 2:9). The phenomenon is called לשון סגי נהור in rabbinic literature. It means "the language of a lot of light," with סגי נהור, a lot of light being a euphemism for blind, and the expression itself basically means "euphemism."
A good article on the topic is "Some Effects of Primitive Thought on Language" by Robert Gordis (AJSLL Vol. 55, No. 3 (Jul., 1938) , pp. 270-284) which begins discussing "words of mutually opposed meaning" noting "the existence of a large number of roots that possess mutually opposed meanings either within the same language or in different members of the same group."
What I am suggesting is that along these lines maybe "יום שמתת רשב"י" was intended as a euphemism for his yahrzeit, in which case there could have been a textual error, but not a change in the intent. At the very least, in keeping with the text criticism principle lectio difficilior*, the more difficult reading is preferred, this possibility should have been explored.
That said, I realize that there is no ancient tradition that R. Shimon ben Yohai died on the 33rd day of the Omer, but the question here is whether the 16th century kabbalists of Safed considered that to be his yahrzeit or not.
*This was noted by Jeffrey Woolf at My Ober Dicta.