I have a situation where I need to upgrade the existing service from 200A, 240V, 1ph to 400A, 208V, 3ph. I'm curious to know how people handle feeding the existing 1ph panelboards when you change phase like this. Would it be possible to keep the original 200A, 1ph main panel and back feed it from a 2-pole breaker from a new 400A, 3ph panel? The service is for an office which mostly has receptacle and lighting loads. Thanks for any help sent my way!