Once upon a time we discussed (see here) the Pri Megadim's chiddush in his Pesicha haKolleles that where the Chachamim added parameters to a mitzvah d'oraysa they uprooted any possibility of a kiyum d'oraysa absent those parameters. For example, m'doraysa a sukkah need not be large enough to fit a table inside; m'derabbanan it must be. Tosfos (Sukkah 3) suggests that if one uses a sukkah is too small to fit a table inside not only has one failed to fulfill the mitzvah of sukkah as dictated by the Chachamim, but one gets no no kiyum mitzvah d'oraysa either.
I have not looked into this at all, but off the cuff I was wondering whether this rule should apply to parshas zachor as well. M'doraysa there is no requirement to read parshas zachor this particular shabbos. According to many, if not most Rishonim, there is no need to ever do a formal krias hatorah to fulfill zachor -- one can simply "remember" Amalek through some verbal declaration (Minchas Chinuch discusses whether one can fulfill the mitzvah through hirhur). However, the Chachamim instituted that our fulfillment of the mitzvah of zachor be done by reading a parsha this Shabbos. Does that mean, based on the Pri Megadim, that one cannot fulfill the mitzvah d'oraysa in some other way, e.g. by hearing the parsha when Ki Teitzei is read in the summer? Or would you say that the Chachamim did not alter the kiyum d'oraysa of zachor, but simply added an additional kiyum mitzvah to listen to the reading of zachor before Purim?