Use existing 200A panel as temporary sub-panel off of new 320A service panel
Yeah, that's actually pretty routine and we often recommend it as a permanent configuration. 

Presuming the old panel isn't a dangerous pile of junk like Zinsco or Federal Pacific or full of Challenger breakers, or fine but obsolete panel like Pushmatic.  Then feel free to replace it with a modern panel, and if you do that, that is the time to think generator. 

Since both panels are in the same building, the "main breaker" on the old panel is superfluous.  The only exception is if you fed it more than 200A, but that's unlikely so I won't comment further on that scenario.

Nowhere is it written that you must supply 200 amps of feeder to that subpanel.  What is written is that the wire/cable from main to sub must be protected by a breaker sized to protect the wire.  For instance if you ran #6 wire to feed the subpanel, its run would need to be protected by a 60A breaker in the main panel.  

So you have the option to save some $$$ on breakers and fat wire if you don't really need 200A down there.  

On the Knob &amp; Tube wiring, there is nothing wrong with K&amp;T per se.  Slap an AFCI breaker on it to catch the usual failure mode (arc faults) and I wouldn't worry further about lighting circuits.  Focus on modernizing receptacle circuits, since modern loads can pull pretty hard.