Q:

Use existing 200A panel as temporary sub-panel off of new 320A service panel

Currently I have 200A electrical service that's brought to my house above ground and into the basement on the right side of the house, which feeds a 200A panel. I'm in the process of getting my service upgraded to 320A, which will be brought in underground and on the opposite side of the house.
It's an old house with mostly knob and tube wiring, so I will be eventually upgrading the electrical wiring throughout the house. But for the time being I need to complete this 320A upgrade (I'm working with the local electrical utility company, and have a permit which expires in a few months), while maintaining my existing electrical wiring. Since the new service will be on the opposite side of the house, I cannot simply re-wire all circuits to the new panel. What I'd like to do is temporarily use the existing 200A panel as a sub-panel to the new 320A/400A panel. The 200A panel obviously has its own 200A master breaker, so in theory it seems quite possible to wire the 200A panel as a circuit off of the new panel, assuming the correct gauge heavy duty wiring is used.
Is this temporary use of the existing 200A panel as a sub-panel possible? What steps would be involved to do this correctly?
Thank you!

A:

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 & Tube wiring, there is nothing wrong with K&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.  

