My impression is that Fred Franz was never sold on Rutherford’s teaching on the “superior authorities” in Romans 13 and that he was eager to change it once Rutherford died. Why he had to wait until 1962 is a bit of a mystery to me. I can only imagine there was resistance from Knorr or from someone else in a powerful position, or perhaps that Franz himself saw some use in waiting.
I think it’s interesting that the NWT in 1950 already inserted the phrase “relative positions” in Rom 13.1 which lends itself to the new understanding that the “superior authorities” are governments rather than Jehovah and Jesus. This makes me suspect the new light was already in the pipeline as early as 1950. It still leaves the question why wait until 1962 though?