I came accross the following exchange on the Internet and am not sure what to make of it seeing that both women are expert native speakers of English. Needless to say, no more was said about this further on in the forum.
My guess is, Maryanne is right, but is she really?
Maryanne Khan says:
December 22, 2011 at 4:16 pm
not to be mean, but I noticed that you said “If you would replace the interrogative pronoun (who or whom) with him, then you should use whom” when the subjunctive case is appropriate. “If you replaced . . .” or “If you were to replace.”