If Kartik didn't like his co-star or the script of the film then he shouldn't have signed on to it. Creative differences is what's normally cited before you start filming. Sure it happens after and in some cases a star is big enough or they've invested enough money they let the star have their way, or the star isn't powerful enough and the director/producer has their way. In this case it looks to be a power struggle and KJO is willing to swallow 20cr and recast.
+1
This is exactly what I think happened. He signed onto Dostana because it was his entry into the Dharma camp. Meanwhile, his star started rising and he got a lot more offers. Even nepo babies like Sara and Jahnvi were drooling over him. It would inflate anyone's ego.
He must have gotten cold feet for sure about playing a gay character especially when he had better and more conventional offers available. He probably thought he can make them change the script so that he doesn't have to play gay anymore.
It was a power struggle and Kartik was playing hardball by not giving dates and causing problems so that they would relent and let him have his way. Since filming had already started, all the power was in his hands. And to be fair, BW actors interfere in scripts all the time and changes happen during filming sometimes. If a bigger star or one of his favorite nepo babies had done this, Karan would have just given them what they wanted and finished the film even if he wasn't happy about it. But KJo would have been pissed that this new guy has such a huge ego and was trying to play politics and arm twist him into doing what he wants. He probably got sick of the tantrums and kicked him out and took a loss rather than give in to Kartik's demands.
10