Another interesting literary area to explore are the myths, especially the Greek/Roman ones. I'm sure their covered really well on Wiki.
The problem with those is that they don't fit the OP's specification:
And all authors have to be English native speakers (so don't bring up Kafka's "Metamorphosis").
Which is a bit disappointing, as otherwise I'd suggest Kit Reed's (if memory serves) "Sisohpromatem" as a matching tale. (It can be found in volume II of Moorcock's
Best SF Stories from New Worlds.)
It depends, too, on what sort of metamorphosis we're talking about here. Gully Foyle undergoes a metamorphosis, after all, in
The Stars My Destination; Charlie Gordon does so in "Flowers for Algernon" by Daniel Keyes; yet I think this is not the type of thing you're looking for. You also have protagonists undergoing a rather different type of metamorphosis (both physical and mental) in Michael Moorcock's early novel,
The Blood Red Game (a.k.a.
The Sundered Worlds) -- one which, it is hinted, will eventuall be experienced by the entire human race. Then again (save for your other statement about being human at the beginning) you could go for Asimov's "The Bicentennial Man", wherein a robot undergoes a metamorphosis to become fully human, even at the cost of mortality. Harlan Ellison has dealt with the subject more than once, in such tales as "Shattered Like a Glass Goblin", "The Place with No Name", the comic "Gnomebody" or "I'm Looking for Kadak", the stark "All the Sounds of Fear", the poignant "Nothing for My Noon Meal", or the pyrotechnic "The Region Between"... to name only a few (it is, in one form or another, a fairly persistent theme in Ellison's work, metamorphosis and/or transformation).
The problem isn't going to be a
lack of sources, but an
overabundance, really....