As far as I know, all regions were still using one SR judge when I graduated from HS in 1969. When I started teaching in 1974, they were using three, so somewhere in those years.
I saw a very talented perennial 1st division Waco Richfield HS band use an electric bass and rock n roll music on the field for the first time in 1974 or 75. It was still a very fine band, but the judges, expecting a military show, hated it and gave a 4.
Not sure of the year they started splitting regions, but as population centers shifted, many urban and suburban regions became unwieldy and huge compared to the more rural regions, so new regions were drawn while others were re-drawn.