According to Today's Bombay times,
Arjun was finalised to play the role but he was removed when the team working came to know that he will be participating in the upcoming reality show JDJ on the rival show.
This is what the team had to say about the development -
"Sidhant had to exit the show for the same reason.He was keen on taking up a dance reality show that he was approached for. It becomes difficulty for an actor to balance two shows at a time. They didn't wanna repeat of that with Arjun"
That's the reason why Naman Shaw is coming in and yes guys Sidhant will be participating in JDJ.
Hope this exactly quoted information from the newspaper helped. :)