I was wondering if there were any deities whose roles had been greatly diminished by the Greek pantheon after being assimilated into it?
This answer will be evolving as I add to the list, but first I'll start with the most striking example:
Artemis: A primal form of Artemis, the Lady of Ephesus, was a mother goddess believed to be related to Cybele. Under the Olympian hegemony, Artemis may be said to have transformed from a "Great Goddess" with unrestrained power to the virgin daughter of the king of the gods. (This may be a reflection of the curtailing of the power of Gaia by casting down her offspring in the Titanomachy and Gigantomachy, and extends to all Olympian goddesses, with some small exception for Aphrodite in still having power over Zeus per his licentiousness.)
Cybele can certainly be regarded as having lost stature in that her original form was a "Great Mother Goddess", surely a central figure, but on incorporation into the Greek Pantheon, she became more of a peripheral figure--still respected but not central.
In terms of loss of status, also worth looking at:
- The Erinyes: The Orestia of Aeschylus is a recounting of the pacification of the Erinyes in service of the advancement of civilization. Nevertheless, their transformation to the Eumenides is demotion both in terms of status and power, in that their ancient domain and prerogatives are stripped from them.
Solsdottir wrote a great article on them: The Avenging Furies. I think there's a case to be made that their stature is diminished in the Greek conception when you compare Hesiod's description to Aeschylus' description.
The Myceneans (a pre-Greek civilization) saw Erinus (singular) as a goddess, listing her along with Atana Potnija (Athena), Eluthia (Eileithyia), Paiawon (Apollo), and the Mistress of the Labyrinth, Potnija Dapurito. Tablets in the Linear B language list her alongside the others without making any distinction, and one records offerings of oil to all of them.
SOURCE: The Avenging Furies
Note that in the Mycenean conception, Erinus had equal status with a proto-Athena and proto-Apollo.
First off, this page you linked to cites Robert Graves as her main source, and Graves' Greek Mythology is not exactly a reliable text. (He believed that a goddess religion had been pushed out by a male-dominated pantheon, and was not above inventing evidence to prove his point. He called it "poetic truth".) You will notice that even the page's author admits that Graves is a dodgy source.
For more reliable evidence about who Eurynome was, check out Theoi.com or Greek Mythology. I suspect Graves based his ideas on the Euryome who was the daughter of Okeanos, the primeval Ocean, and was cast out of heaven with her husband, Ophion, by Kronos and Rhea.
This is a different story-line than the standard one from Hesiod's Theogony, which goes straight from the first ruler Ouranos through Kronos to Zeus. Since Euryome's husband in this version is Ophion, it probably comes from the Orphics, a mystery cult that was supposedly founded by the legendary muscian Orpheus.
That seems to be as close as she gets to any creation myth. She did have a cult centre in Arcadia, where a wooden image of her (in mermaid form) was kept, and sacrifices offered once a year.
Several different figures also share the name, one of whom nursed Hephestios when Hera cast him out of heaven, and another who married Zeus and was the mother of the Charities.
Paion or Paean, a healing god who appears in Linear B god lists from Knossos and was later mentioned by Homer and Hesiod as a healing deity. (One Knossos list mentions him alongside the Lady Athena, a form of Ares (Enyalios) and Posedion. So he was important enough to be listed with three future Olympians. As Fritz Graf puts it: "in Homer Paeon seems an independent mythological person, whereas in later Greek, Paean was epithet of Apollo the Healer to whom the paean was sung and danced." (Apollo: 110)