If you don't take admission on the basis of JOSAA seat with in the stipulated time, then automatically the seat will be count vaccant and will be forwarded to the other students in the next round. So if you get a better seat through CSAB you can obviously opt for it and there will be no problem.

To know more about JOSAA visit the link below:

https://engineering.careers360.com/articles/josaa-counselling

To know more about CSAB visit the link below:

https://engineering.careers360.com/articles/csab

I hope this answer helps. All the very best for your future endeavors!