3 wire sub to main panel upgrade
When the building was built code was probably 3 wire ( this was the standard for decades). What you have was quite common until the1999 NEC code change.
A 200 amp feed would normally be in conduit and if metallic that could be used to meet today’s code.
If it is a direct buried feeder and you feel the need to update it a separate ground wire is now allowed, in most cases a panel that was 3 wires will have the neutrals and grounds connected or even on the same buss this will need to be changed to isolate the neutral from ground.
Also every receptacle will require GFCI protection to bring it up to current code but this is not required as your existing configuration is grandfathered.
If you really think it is unsafe add the GFCI protection.