Im getting ready to upgrade and relocate my 200 amp service to a 400 amp service about 10 feet away away from original location. A new 400 amp meter base will be back to back with 2 new 200 amp panels. My question is, can I take out the guts out of my 200 amp panel and use it as a junction box to splice and continue my branch ckts to the new panels location? There are about 20 ckts and I'd hate to put up a junction box for each ckt. What is the minimum legal requirements?

Im using 4/0-2/0-4/0 aluminum double tapping off the meter base to each panel and using a #4 GEC.

Well you wouldn't need a box for each circuit. You'd be able to put 3 or 4 circuits in a decent sized splice box.
Where do most of the circuits come from? Can you pull any of them back to the new panel location?

You can use the panel as a splice box, but IMO this is a cheesy way to go. You'd also have to screw the cover shut

None will be able to pull back to new location, good thought; I am going to plan on using about 6 to 8 conn boxes to splice the connections. Do you see any problems using the 4/0-2/0-4/0 with a #4 GEC and double tapping off the meter base, which will be located directly behing the panels outside, to get to each 200 amp panel?

I see some talk about a 320amp meter base/service but is called a 400 amp service; what's the reason for this, why not just a get a true 400 amp service?

You can ONLY double tap off the meter if the load side lugs allow it, such as double hole lugs. This is very common.

A residential 400A service is 320A continuous, and 400A max. It uses a standard plug in meter instead of CT's (current transformers) that a true 400A commercial service would.

Thanks Stan all help is greatly appreciated