There were no changes in the powerhead, the 50:1 was a mixture change made across the board….
I would stick with the 24:1 mix, a little extra oil provides a nice safety net in cases where the engine might encounter some heavy stress.

You have to remember from the mid 1950s to the mid 1960s the engineers were still learning about lubrication. The first synthetic oil by Lenard Oil in Michigan was a total disaster. A lot went into research and development, but the quality of oil available to the public ran from super to useless. We tested a half a dozen oil additives all guaranteed to reduce friction, improve milage, prolong engine life and make you a better lover. One thing they did have in common is, they did lightened your wallet. One, Bardoil did show results. It put a hard film on bearings and cylinder walls that eliminated metal to metal contact. We did see a slight increase in horsepower after a few tanks of fuel. Further testing revealed the film continued to build until the engine seized up. Evinrude recommended you use it only in every fourth tank of gas. So the outboard oil developed by Evinrude was required to be used by them to work at fifty to one. They were immediately challenged by the legal departments of several oil companies who said there oil was just as good. The result being that Evinrude could recommend but not require. So, to answer your question, in 1963 there was still a lot of questionable oil available. In 1963 Better oils were on the shelves. The motors did not change – the oil did. . . 😆

Compare the parts lists for the 1963 and 1964 28hp motors and report back with the results. I can wait. Might as well compare the tolerances too, while you are at it.