As replacements joined the cast, the original song list was altered, with some additions and deletions.
For example, when the cast comprised Georgia Brown, Larry Kert, Nancy Dussault and Hermione Gingold, "The Ladies Who Lunch" and "The Miller's Son," not in the show originally, were added to the production. Both were sung by Brown.
The duet of "A Boy Like That / I Have a Love" was deleted and replaced by a duet version of "Waiting for the Girls Upstairs" from Follies. This was sung by Kert and Brown.
Both women's parts of "Getting Married Today" were performed by Dussault with Brown was not in the number at all.
"I Never Do Anything Twice," originally performed by Millicent Martin, was given to Gingold when she became the narrator. It was her only musical number. (Ned Sherrin, the original narrator, had no musical numbers.)
At some point after Larry Kert joined the cast, "Marry Me a Little" was deleted and replaced by "Being Alive." For at least for the few months of Kert's run in Side by Side, he did sing "Marry Me a Litte." (“Marry Me a Little” seems not to have been in the original London production but it was included in the Broadway production, replacing “Being Alive” from the London production. So when “Being Alive” was put back, that was a bit of a return to the original London version.)
When Burr Tillstrom joined the production as narrator, the previously unknown Sondheim song "The Two of You," which Sondheim had written in the early 1950s for Kukla, Fran, and Ollie (and, of course, Tillstrom) was added to the show. The song had never been performed publicly. It seems that the song was sung primarily by Nancy Dussault, with Tillstrom singing just a few lines (as Kukla and Ollie), though there may have been periods during the run or on the tour (in which Tillstrom also appeared for a time) when Tillstrom sang the song himself.