It's not clear you have to change the panel at all if you don't want to make use of the extra service capacity.

If you do, then you can do one of the following:

1. Install a 400A panel and scrap the existing.
2. Install a small 400A panel and feed two 200A subpanels (you may be able to rework your existing panel into this capacity)

There's nothing to "balance", you just don't want to exceed the capacity of any one panel.

Thanks. If I do this, I'd rather not have to reconfigure the existing 200A panel, because that's just paying someone to do something that's unnecessary.

Option 2 is what I'd likely do, and perhaps not even have a second 200A panel installed unless that's necessary for some reason.

They are probably wanting that because they are going to run 350 KCMIL which won't fit the lugs in your 200 amp panel. There are solutions for that but the POCO probably doesn't want to bother with them (popsicles).

Do you have an all in one, that is a single box with the meter on one side or on top, and the breakers on the other side, or is the meter in a box of it's own?

If you have the latter, it would be a simple matter to just install a 400 amp meter base and connect it to your current panel.

Meter is in its own box outside. Cable through wall/foundation to breaker panel.

Poco wasn't demanding anything other than they apparently don't do underground connections for less than 400A. I doubt they care what I do on the other side of the meter (well, so long as it's safe). I was just wondering what my costs are going to be for that, but it sounds like it should not get too expensive for that part.

400 amp equipment generally costs many times more than 200 amp or less equipment. For example, a 400 amp all in one costs me 8 times as much as a 200. I don't know how much a 400A meter base is, maybe someone else will chime in. Also, expect labor to be higher on a 400.

Around here the local POCO supplies 400A meter bases, even if yours doesn't the cheapest and easiest way to go would be to install a 400A meter base and then feed (2) 200 amp panels off of it. In residential this setup is very common, at least where Im at.

As others have already mentioned you can buy quite a few 200 amp panels for the cost of a 400.

Yes, the installation of a 400 ampere meter could supply two panels of 200 amp rating IF both of those panels have a main disconnect/breaker that disconnects all loads in the panel. Up to six such panels could be supplied. Read NEC 230.71 (A). When using more than one panel, the requirements of NEC 230.90 (A) Exception No. 3 must be met.

You can use that 6-throw exemption provided all the panels they are in are grouped together.

That is how the typical 320/400A residential service is done around here.
Two 200's side by side.

No one I know uses a true 400A service with one panel in a residential setting. Makes no sense if you ask me.
Two 200s are cheaper, easier and provide all the breaker space you need without needing sub-panels (in most applications).

The house I'm building now has a mongo 400A disconnect and then 2 200 breakers to feed the subpanels (and eventually there will be 6 panels total).

Thanks everyone. Back to the original question, though. If I put in a 400A disconnect with a feed to my existing 200A panel, do I need to feed a second 200A panel at all? Maybe I will one day, but is there any reason I'd need to now?

You probably do not need the 400 amp disconnect. Your jurisdiction may have a specification regarding the maximum distance from the meter to the first disconnect. If the distance from the new 400 amp meter to your existing panel does not exceed that limitation, just power the existing panel from the meter with conductors suitable for 200 amperes. That's it.

The 400 amp disconnect feeding only your current panel should be sufficient.
Your current panel will now be a sub panel. It will need a four wire feed. It will need the ground and neutral separated. The grounding system will need to move into the new disconnect.

If he doesn't care to draw more than 200A, he doesn't need to do anything. The 200A main breaker as disconnect is not larger than the service fed in. As pointed out by joefixit, the worst thing that is going to happen is that someone tries to run from the new meter base with wires too big to fit the lugs on the old panel. The easy out is to just use smaller wires (of sufficient ampacity for the 200A disconnect).