I believe that the Pope can, in theory, change canon law at whim. It would, however, be an abuse, and potentially (on his part) a mortal sin in many cases. I like Yeshua’s allusion to the power of ordination. A bishop could come up to me tomorrow morning as I walk along the sidewalk and ordain me to the priesthood; the ordination would be valid, but it would be an extreme abuse of his authority (“do not be hasty with the laying on of hands” as Paul instructed Timothy). We must remember that Jesus gave Peter the “keys to the kingdom” and the power of binding and loosing. As surely as the Pope can bind the Church to follow a certain canon he can loose the Church from the same. That being said, the Pope can’t do ‘whatever’ he wants…there certainly are restrictions. He is bound by Sacred Tradition, and in particular, by infallible declarations of faith promulgated by his predecessors and the ecumenical councils. The pope can freely change canons pertaining to discipline, but not those pertaining to doctrine.
A good example would be the case of clerical celibacy versus women in the priesthood. The Holy Father could, in theory, wake up tomorrow and rescind the canons pertaining to clerical celibacy allowing the Latin Church to ordain married men; he could not, however, suddenly admit women into the priesthood. John Paul II (of blessed memory) made it clear that the Church (including, of courrse, the Roman Pontiff) has NO AUTHORITY to ordain women to the priesthood.