So if you move your capital, you can build in the new area, not in the old one, but it doesn't affect your previously built tradeposts? Are they still able to be upgraded, claimed, etc?
Yeah you will keep your old trade posts
Based on those notes, I take it the Special Unit function and any related modding changes are only going to available to people who purchase RoI?
No, the patch notes are stuff not locked to the DLC.