When Manilow recorded it, there had just been a big hit by Looking Glass called "Brandy (She's A Fine Girl)", so hence the change to Mandy.
I like the ‘Andy’ story better …
Elliott Lurie of Looking Glass still sings “Brandy (You’re a fine girl) in LA clubs occasionally. He went on to head MGM music. He took the name from some crazed hippie chick who wasn’t impressed that he wrote a song about her and changed her into a barroom waitress. Magnificent song. Far better than that “Mandy” thing.
“Brandy (She’s A Fine Girl)”
That is fascinating! Brandy hit #1 the summer I was 14, it was a one hit wonder for that band.