Sorry,
My brain when dead on this one.

Situation: Original living quarters (900 sq.ft.) with garage, built in 1995, and had its own main 200 amp. panel supplied by an underground service.
A separate detached house was built in 1999 on the same piece of property within 20' of the original living quarters. The newer home has a 400 amp. main panel. The original main 200 amp. panel is now converted to a branch panel in the original detached structure and is being fed by a 60 amp. 240v. circuit through plastic electrical conduit run underground between the two panels. There is no ground run from the main 400 amp. panel to the 60 amp. branch panel through this conduit (only the two hots and a neutral). The branch panel is relying solely on its original grounding connections to its water piping and to its Ufer ground.
Is a grounding conductor required from the main panel to the branch panel since this is a detached structure (and yes, the neutrals were still tied to the grounding buss in the branch panel - thank you sparky).