"Years ago I saw a production of Nine at NYU in which the character of Liliane La Fleur was played as an outrageously flamboyant gay man, replete with beads and large feather boa during Folies Bergère. It worked fine.
If I remember correctly, this production incorporated ideas from early drafts of the script before Tommy Tune became involved and decided that Guido would be the sole male character in the show. Arthur Kopit gave special permission for this and the other book changes; this version of the show isn't commercially licensed."
The edition of the script for Nine that I own does have an option for LaFleur to be played by a man, as well as a few other smaller parts, so that aspect at least does not require special permission from anyone, unless Samuel French has changed this in the years since I bought the script (which was back in undergrad so like 13 or so years ago).
If Mama Morton was a boy, not a drag queen - an ACTUAL boy, I think it would make Chicago a whole lot darker because three men would be looking to have Roxie Hart in their own way. Almost makes he sympathetic, even though she's still a dick.
An actual boy like a child, or just a male adult? Because neither would make sense within the text, but the first suggestion would just be frightening.
"Impossible is just a big word thrown around by small men who find it easier to live in the world they've been given than to explore the power they have to change it. Impossible is not a fact. It's an opinion. Impossible is not a declaration. It's a dare. Impossible is potential. Impossible is temporary. Impossible is nothing.”
~ Muhammad Ali
I've seen plenty of productions of The Fantasticks where one or both of the parents are played by women. Usually Bellamy, and I think every time I've seen it, they turned it into a romance between the two parents to contrast with the young lovers. Also, not sure if it counts, but the women's roles in Pacific Overtures, written to be played by Men, now are frequently played by Women.
If and when I ever direct Assassins, I want to cast the Proprietor as a Woman and have the same actress play Emma Goldman. I'd like to claim this is wholly original, and I know I came up with it on my own, but I recently found out a British production did just that several years ago.