This is the plan for future. Most of the new generation ships can be modified to carry Nirbhay missile also.Correct. But considering that navy designers have overall done a good job with Kolkata class, I would expect them to load the ship with similar no of Anti ship missiles as destroyers from other countries or at least leave space for future up-gradation. In case they didn't leave space in Kolkata class, they would have definitely fixed that in Vishakhapatanam class. Since Vizag class did not go through any major structural changes, it can assumed with high probability that Navy designers have already taken care of that aspect. After all if we can see the issue, the professionals must have thought it through.
Plus it's not only the destroyers, our frigates are under armed too. So there is a pattern which indicates that more Nirbhay/Brahmos launchers will be added a later stage.
In the end, it's just a guess. @vstol Jockey What is your opinion on this?