Thinking about opening a demat account for trading purposes. So is it important which service provider I use to open my demat account? If so, then which is the best service provider or bank right now, and why?

Hi 
I purchased car insurance from Policybazaar, but I forgot to add personal accident cover (PA cover). So can I upgrade my current policy to include PA coverage? Will the policy number be changed ? Or do I need to buy the new policy and cancel the old one, would that result in problems with RTO registration? I cannot purchase PA coverage separately.