It is also the first book in the Janus series of 3 books.
The Goodreads description names the race Iftcan not Ilfen so you weren't far off.
Naill Renfro and his mother were just two members of the flostom that had washed up in the Dipple, the vast refugee camp on Korwar that had taken in the dispossessed of the destroyed worlds, then forgotten about them.
In order to buy his mother the drugs she required end her days in oblivion, if not peace, Naill sold the last thing of worth he had left; himself. He knew the planet Janus would not be a pleasant world, else there would have been no need for indentured labour. But both the service and the planet are living hells, right up to the time Naill found a pretty tangled in the roots of a felled forest giant.
Then he found the true meaning of hell as a disease overwhelmed him, leaving the Garthmen to abandon him to a strange fate as he is transformed into a new form: green skinned Ayyar of Iftcan, a civilization that had died more years ago than either Naill or Ayyar cared to think of...
The following description does a nice job of descibing the story in more detail and matches your points.
This being Norton, that doesn’t mean what it might mean in another story. He signs on as a slave laborer, and ends up on Janus, a forest world which has been colonized by a fanatical religious sect. The sect is all about sin and repression, and about eradicating the forest. The forest fights back by infecting some of the colonists with the “Green Sick.” There are also, Naill learns in short order, treasure troves of alien artifacts which turn up around the settlements, and which are destroyed with great ceremony and ferocious thoroughness.
As soon as Naill learns about this, he finds a trove, and is irresistibly drawn to the items in it, especially a kind of shiny tube. He tries to hide it when the treasure is destroyed, but is caught. And then he becomes sick.
The artifacts are the cause of the infection. And, Naill discovers when he comes to, the disease transforms its victims into aliens with alien memories. He is now a hairless green goblin with huge pointed ears and eyes that can’t tolerate sunlight, and he has the memories of an ancient alien warrior named Ayyar. He also discovers that he physically cannot tolerate humans. Even the sight of them causes visceral revulsion. This feeling is mutual: humans run screaming from the green monster.
Tor.com, Going Green: Andre Norton’s Judgment on Janus
I found this by Googling
"green sick" scifi which revealed the Tor link.