Q. On which hand does a lefty hold the lulav and the esrog? If he reverses does he also change the hadassim and the aravot?
A. It would depend whether one is Ashkenazi or Sephardi. Shulchan Aruch (651: 3) rules that even a left handed person holds the lulav in the right hand as everyone else and that is the Sephardic custom. Mishna Berura (18) quoting Tur, explains that only regarding tefilin which is a Biblical mitzva a lefty reverses. However, lulav on the right is only Rabbinical, so one maintains the custom of all others.
However the Ashkenazi tradition follows the Rema (ibid.) and the Rosh, who maintain that a left-handed individual holds the Lulav in his more important hand, the left hand, and the esrog on the other.
See Mishna Berura (ibid. 19) regarding after the fact, if one mistakenly did the opposite, he should repeat without a brocho.
However, the setting of the haddasim and aravos is the same as everyone else.
Rabbi A. Bartfeld as advised by Horav Shlomo and Horav Aharon Miller Shlit’a