first off, i should mention that blazblue extend is about to die pretty soon, what with the new blazblue chrono phantasma JP on consoles coming out very soon.
but with that said, if you're still going to be playing ragna in both CSEX and CP, then you should focus on neutral gameplay, since that is pretty much intact between the iterations. the best way to do this is to just fight against people online. study what distances from the opponent work, and keep in mind the tactics that work as you fight more people. you can use the ragna wiki if you want some sort of guide on how to play ragna.
as for combos, it's not worth learning them anymore due to the fact that CP ragna has completely different combo routes. but if you still want to learn them, look at the ragna wiki for optimal combo routes.