In my memory, the rotation ended for all positions, including PO, at the same time. I could be wrong.
I remember the "adjustment" clearly because I had just rotated into the position of PO in 1983 and was disappointed to lose the position before I could make what I perceived then to be important changes in the way the congo was run (the man who rotated into the position the year before was totally disorganized).
But it was clear the rotation system wasn't working - we had guys conducting the school who couldn't read or communicate, and they were up there counseling people every week on their reading and speaking skills! We had secretaries who could not write a letter and didn't know how to file (or didn't bother). We had WT overseers who clearly hadn't read the article under discussion, or did not understand what it said.
Ending the rotation was the only move the WTS could make. As someone said, above, they weren't too concerned about a permanent PO because the position was largely ceremonial, stripped of the power that the old Cong Servant had. But over time, the PO became a powerful position, one in which the holder of it set the tone for the entire BOE and the congo itself. There are many mean-spirited, egotistical PO's running congos now and it is nearly impossible to get rid of them unless the majority of the BOE forms a political action committee and takes him out.
I saw all this and dreamt of reform. It was many years later when I realized the problems were systemic and beyond repair.