Olaf Stapledon's Star Maker (1937) is narrated by a human who becomes noncorporeal, encounters various other disembodied minds, and ponders this development as part of the evolutionary destiny of sentient species. They can still interact with the material world - including feats of galactic-scale engineering - but mainly zip around at superluminal speed and think about the wonders they encounter. In several alternate universes, there are different outcomes, but "an evolutionary process crowned by an awakened cosmical mind" is what happens in the narrator's home universe: sentient species eventually become noncorporeal and all merge into a single group-mind.
This is an acknowledged influence on Clarke in 2001 and several of his other works, such as Childhood's End, as well as on C. S. Lewis' space trilogy which features the incorporeal eldila. Lewis objected to Stapledon's presentation of the titular Star Maker (i.e. God) but was still interested in the imagery of the book.
In the story, the narrator imagines being able to see through the body of the Earth, and then perceiving it from the perspective of an observer in space. All of a sudden -
The very ground on which I had been sitting was gone. Instead there lay far below me an insubstantial gloom. And I myself was seemingly disembodied, for I could neither see nor touch my own flesh. And when I willed to move my limbs, nothing happened. I had no limbs. The familiar inner perceptions of my body, and the headache which had oppressed me since morning, had given way to a vague lightness and exhilaration.
Discovering himself able to travel through space, he resolves to answer the fundamental questions of life on behalf of his species:
I now seemed to my self-important self to be no isolated individual,
craving aggrandizement, but rather an emissary of mankind, no, an organ of exploration, a feeler, projected by the living human world to make contact with its fellows in space.
He makes telepathic contact with (corporeal) aliens, and forms a sort of joined consciousness with one of them. They continue to explore together, encountering other aliens and linking them into the group-mind:
[A]fter a while we came in psychical contact with another group of
cosmical explorers, natives of worlds as yet unknown to us. [...] When we had encountered many more such groups, we realized that, though
each little expedition had made a lonely start, all were destined sooner or later to come together. For, no matter now alien from one another at the outset, each group gradually acquired such far-reaching imaginative power that sooner or later it was sure to make contact with others.
In time it became clear that we, individual inhabitants of a host of
other worlds, were playing a small part in one of the great movements by which the cosmos was seeking to know itself, and even see beyond itself.
The narrator's body is presumed still present on contemporary Earth while his noncorporeal transcendent self is elsewhere - travelling in both space and time. He comes back to it at the end. So this is not a one-way transformation, although he has achieved a higher consciousness on return. Still, the idea that this state is the destiny of humanity in general is present; he's had a foretaste of it, and is clear that in the distant future of our universe, our descendants ought to be like this if they survive.
From some other fragments of description, I think it's fair to class the narrator's experience as becoming a noncorporeal being, rather than simply projecting his mind telepathically outwards. He experiences vast timescales "far beyond the normal age" that an ordinary human life could sustain, accesses the memories of others as if they were his own, and engages in mental feats as "a mind of higher order than any of us in isolation".
Stapledon's earlier work Last and First Men (1930) includes various iterations of human evolution, including the Eighteenth Men, who form a supermind. They are, however, not lineally descended from present-day humanity, and are corporeal. I think that Star Maker is a better match for the question.
