The form of address is very significant in the novel and it implies a shift in formality and denotes an acceptance of party doctrine and fellowship. Whenever you address someone as "Mrs. so-and-so" the "Mrs." belongs to the identity of the proper noun which follows it. For example, Mrs. Pearson, Mrs. Beasly, etc. It also denotes marriage and that that individual believes in marriage and all that that implies. The party of 1984 wants to subvert marriage to all aims and desires of the party and of the brotherhood.
When one replaces "Mrs." with "Comrade" there is a public declaration that the speaker identifies with party doctrine and that it is an address between one "brother" and another "brother" in this great thing that's called "the party."