Replacing the Hadasim and Aravos on Chol Hamoed: 
How to replace the Hadasim and Aravos: One is not to stick the new Hadasim and Aravos into the knot that is over the Lulav. This causes leaves to shear and can invalidate the branch, as well as invalidate the status of the knot. Rather one is to undo the knots and then place the new Hadassim/Aravos, and then retie them together.
 Sefer Haminhagim p. 145
 See ibid footnote 566.
The Rama 654/1 rules it is customary to switch the Aravos on a daily basis and that doing so is Hiddur Mitzvah.
 M”B 654/4 in name of Bikureiy Yaakov 654/4