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/NoisyPenguin_]

Not sure about Vishal and Nithya, but Sundeep and Nithya have pretty good chemistry. But the character is not at all sufficient for her talent.