If the subfeeds / subpanels will have increased loads connected to them [like 50% or more of their maximum capacity], plus the loads will be running at the same time ,along with 3 hours or more [LCL], than this new load calculation might very easilly exceed the existing service capacity.
Is the existing service 200 amps?
If this Client plans to draw high loads and expand in the future, the 400 amp service is a valid concern.
Calc the project out, then figure out what is the best way to go.
Scott " 35 " Thompson
Just Say NO To Green Eggs And Ham!