This sounds like a project which should have been designed by an EE [just my $0.02], but since you are willing to overtake this task, let's see if we [all in this Forum] can assist enough here.
Leaving the existing 200 amp 1 phase 3 wire panel and making it a subpanel is a good idea - provided this will be adequate for the loads [I'll expand on this].
I guess this was originally the Main Service panel??? If so, do as was suggested and be sure the Grounded Conductor's bus is Isolated from the enclosure [AKA not grounded in this panel]. Be sure to remove any EGCs from the bus and place them on a grounding bus strip [may be easier to place the Grounded Conductors on a new Isolated Bus and leave the existing bus with the EGCs as is].
As to leaving the bus kit as 1 phase 3 wire, this [as far as I know] is NEC compliant. It just brings up imbalance issues [and is strange to us "Veteran 3 phase guys"... humor].
If the loads which shall be connected to this panel would require 3 phase, or if there would be an advantage to having 3 phase 4 wire circuitry, then change the bus kit to a 3 phase 4 wire one and add the 3rd Ungrounded Conductor to the panel.
This will all be determined by what the connected loads need.
Per the SCA quoted by the PoCo [Utility Engineers], this would be the minimum 3 phase "Bolted" available fault current level at the Service.
If the subpanel in question is far enough away to reduce the SCA level far below 10KAIC, then you may use the 10KAIC breakers in the subpanel. If the SCA is 10KAIC or above at the subpanel, the existing 10KAIC frames would need to be changed out for 22KAIC ones [unless you try for a Series Rated Type System, which will be really difficult to accomplished and may become extremely expensive if the existing panel is older equipment].
The Subfeeder OCPD would need to be rated above the SCA quoted level found at the main service [not less than 22KAIC]. This could be accomplished by using RK5 type fuses in the switch in the gear section. RK5s are rated 200KAIC.
If a MCCB is used in the switchgear section, it would need a rating of not less than 22KAIC.
Let us know if these items help and if you have further questions.
Scott S.E.T.
