The song that Tim Rice revised was "The Lady's Got Potential", sung by the narrator "Ché" (who at the time was directly identified as Ernesto "Ché" Guevara) and opening with a discussion of his development of an insecticide (which really happened; the insecticide was named Vendaval but it flopped). To quote from the original lyrics on the concept album:
"The Lady's got potential, she ought to go far
She always knows exactly who her best friends are
The greatest social climber since Cinderella
But Eva's not the only one who's getting the breaks
I'm a research chemist who's got what it takes
And my insecticide's gonna be a best-seller
(Che brandishes a large bicycle pump type insecticide sprayer, and is joined by a shrill and enthusiastic trio of girls for his fly-killing choruses)
"Just one blast and the insects fall like flies!
Kapow! Die!
They don't have a chance
In the fly-killing world
It's a major advance
In my world
It'll mean finance
I'm shaping up successful capitalist-wise."
But the surprising image of Ché Guevara as a research chemist was cut as a distraction, and Rice cut the song and added "The Art of the Possible" instead to show the rise of Juan Peron. In the movie, as the question states, Rice wrote new lyrics for "The Lady's Got Potential" that build into "The Art of the Possible". Or, as the Wikipedia entry states:
Elements of the original plotline on this album were removed before the show was staged in London in 1978. "Che" here was much more explicitly based on Che Guevara, including a subplot about Guevara's failed efforts to market the insecticide Vendaval, most significantly highlighted in the song "The Lady's Got Potential". The track was cut from the score and a new song written to include the key plotline of Juan Perón's rise to power: "The Art of the Possible".
https://www.squareone.org/evita/conlyric2.html
https://en.wikipedia.org/wiki/Evita_(album)
