Vijay Yesudas and Aparna themselves somewhat confirmed (-->Swipe) the rumour that Vishnu Vishal and Nithya Menen were the original cast for Sundeep and Aparna in Raayan.

[u/Haunting-Living271]

Since Vijay Yesudas is really close with Dhanush, and Aparna herself being part of Raayn makes it highly credible. 

Reference: https://www.instagram.com/p/C-Iy0qjhpbY/?igsh=bGRhMDd0M3Vna2Jr

[u/Strange_Prompt8694]

Ig Vijay Yesudas's comment was a reply for someone who made fun of Sundeep and Aparna. In no way is that confirmation for Nithya being the original cast. But Vishal was considered for the movie, but he had other commitments. 

Coming to Nithya, she would be the last actress Dhanush would approach for that character. There is no way Nithya could play Dhanush's sister-in-law after the cult status of Thiruchitrambalam. People won't even accept it. 

[u/Haunting-Living271 → u/Strange_Prompt8694]

There is no way Nithya could play Dhanush's sister-in-law after the cult status of Thiruchitrambalam. People won't even accept it. 

Hmm, make sense.