The harmonized occupation codes already do this work for you! Each OCC/OCCLY will always map to a single OCC1990/OCC90LY code using a modal assignment procedure. This procedure is based on a series of technical papers published by the Census Bureau where observations are double coded into the occupational schemes of the current and previous census year (see OCC1990 for more information). Depending on how specific the occupations that you want to study are, you might additionally consider aggregating occupations into broad categories implicit in the 1990 scheme in order to increase comparability across code vintages.