Philip -
I may be wrong on this and perhaps shouldn't even reply, but what happened in earlier Whirlpool/Kenmore machines may apply to the 1/3 and 1/2 HP GEs also.
In the earlier 1960s, Kenmore washers were advertised as having 1/3 HP motors. These were the round motors that could be taken apart for servicing to some degree. At some point later on, 1/2 HP motors went into production across the board. Eventually the motors were redesigned to the permanently sealed, unserviceable square units we see today. This started with the 1 and 2-speed motors in the late 60s or very early 70s. The three speed motor was never re-designed that I know of but was discontinued instead by 1974.
A version of the square frame motor is available as a replacement and still offered by Whirlpool, and it fits basically every belt-drive ever made, 1/3 OR 1/2 HP, round OR square originally.
I would rather have a round motor in a machine originally built with one, but that's beside the point a bit here. I would surmize that the GE horsepower switch was done in much the same fashion, and could even have been encouraged by GE at Whirlpool, since some of Whirlpool's motors were GE built. In fact, many MANY early 1970s WP machines had GE motors in them.
Some of this is on-point and some isn't, but I think the horsepower switch was an evolutionary change that happened to much of the industry, not just GE. You would probably be fine to install a 1/2 HP motor in an original 1/3 application.
Gordon