I can't remember if the Mungojerrie and Rumpleteazer number was sung as a duet when I saw the first national tour in Boston in March 1984, but that change was there when I saw the show in 1992. I knew the actor playing the role at that time.
The quality is not the greatest, but here is a brief video of choreographer Gillian Lynne putting Finola Hughes through her paces as "Victoria, the white cat."
For those of you who may not know Finola Hughes went on to co-star with John Travolta in 1983's "Staying Alive" - a sequel to the popular film "Saturday Night Fever".
She is best known however for the long running role of "Anna Devane" on ABC's "General Hospital". She also played her General Hospital character's twin sister "Alexandra" on "All My Children" and was also on the TNT supernatural drama, "Charmed" as "Patti Halliwell"
Here is Finola performing her solo in the London production of "Cats":

DottieD'Luscia said: "I can't remember if the Mungojerrie and Rumpleteazer number was sung as a duet when I saw the first national tour in Boston in March 1984, but that change was there when I saw the show in 1992. I knew the actor playing the role at that time."
I've never found a definitive answer for when they made the change on Broadway, but it's presented as a duet on the 1986 Australian cast recording so it may have been some time before or after '86. The London production always used the jazzy duet version of the song. "The Ballad of Billy McCaw" was also switched out in London in favor of the Italian aria at some point in the early 90s. It had definitely been swapped by 1992 as there is video evidence on YouTube.
For anyone interested, this video has some footage and more pictures of the OLC designs:
Never knew Mel Bush was in the original cast and that Judi Dench was slated to play Grizabella! I'd love to have heard her "Grizabella the Glamour Cat."
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