Turning 100A Main to subpanel of 200A new main
First your existing panel falls under the NEC it was built under, whatever it was, in 1970 in your area. This is known as the grandfather clause. So it is fine since you are not trying to add to it, just upgrade your service. Now everything you are installing new has to fall under the code in enforcement in your area today. 

So your intent as you have stated is fine. The only thing that I would change in the existing panel would be the grounding. Since you are installing a new 200A main the grounding process must start there. So ground the new panel per the current NEC and remove the bonding screw or jumper from your sub-panel. Install an grounding bus in the  sub-panel and move your ground (bare wires) to the new grounding bus. I can't give you the exact details of how because of lack of information.

Other than that I would say you are good to go. Good luck and stay safe.