Who brought in the changes? The Dravidians (mentioned as Dasyus* in the Veda) or the Aryans, per force.
Both brought in the changes. A friendship requires two people. Both made changes in their ways.
Dasyu (Criminal) and Dasa (servant) have different meanings. And the indigenous people were none of that. To say so is a Christian/Muslim strategy to denigrate Hindus and Hinduism. Dasyus relate to a natural phenomena when the Aryans lived in the sub-polar regions and those regions had a long night lasting two-months. Aryans thought that this is the work of demons. They called these imaginary demons as 'dasyus' whom Indra had to kill each year to bring back the sun.
- In Zorostrianism, Azi Dahhak was the demon and killing him was the work of Thraetona.
- Among the Greeks, The titan Geryon had the cows (RigVeda mentions sun's rays as cows) which Herkles stole. Also Persephone has to remain with Pluto for six months.
The same story is repeated in many mythologies which were influenced by Aryans.
- Among the Celts, Fomori and Fir Bolg are killed and Lug returns triumphantly or Gwyn and Gwythur fight for the damsel.
- Among the Irish, it is the men of Fidga and Labraid of the Swift Hand on the Sword, king of an Irish Hades, whom Cuchulainn goes to assist in the Battle of Moytura.
But what do you know about these things? Your sole informant is either your revealed book or Zakir Naik. :Face Palm: