The biggest thing (and what you omitted) is that the grounds and neutrals need to be separate in the subpanel. Depending on the manufacture of the panel, you'll remove jumpers/screws and/or add additional terminal bars. Either way you're going to most likely need to move wires around on the branch circuits.

A number of companies make a combination unit that is a meter can and a small panel (8-12 spaces, 16 -24 circuits) that have feed through lugs. All you need to do is run the 4 wire feeder from the new panel to the old, and make sure the neutrals and grounds are separated (neutrals must be isolated from the box) at the old panel.

Unless there is a local rule that requires a disconnect at the sub panel it isn't necessary. If the existing panel has a main breaker you can use that for the local disconnect if you want one, but the main breaker in the new feed through panel serves as the required service disconnect and overcurrent protection.

"The biggest thing (and what you omitted) is that the grounds and neutrals need to be separate in the subpanel."

Give the guy a break. He didn't omit it at all:

"wired as a 200amp subpanel (with all the proper neutral/ground separation and so forth)."

Robin,

You could just buy a 200 Amp enclosed circuit breaker for your main at the new location. No need for a panel unless you want additional circuit capacity. Run your four wires to the old panel, Mark them well (Black, Red, White, and Green)at each end before you pull them in, and go for it.