I asked which state in a comment because I wanted to check the dates for the HARN (High Accuracy Reference Network) and FBN (Federal Base Network) re-adjustments in California.
According to GEOCON 2.0 doc, the original HARN re-adjustment was in 1992. The FBN (AKA HARN v2) was in 1998. This document, "AN EVALUATION OF THE HIGH ACCURACY REFERENCE NETWORK
RELATIVE TO THE CONTINUOUSLY OPERATING REFERENCE STATIONS" by Kathryn Milbert, identifies that the California HARN re-adjustment was published in 1992 and the control points were constrained to 1991.35. By "HARN" do you mean data that's actually on the second re-adjustment in 1998, which the National Geodetic Survey is now calling the FBN (Federal Base Network)?
Maybe a geodetic surveyor in California or someone else could figure out the difference using a set of control points in your area of interest and run them through HTDP (Horizontal time-dependent positioning). Using those calculated offsets for several control points, calculate an average, or grid the results.