I'd rather Earth-2 be preserved as it was designed to be- the home of all the Golden Age stories. Superman debuts in 1938 with the rest of the JSA coming over the next few years, Excluding heroes who might "naturally" have extended lives no fountains of youth. By 2020 the active heroes would be mostly legacies and the originals would be available for Times Past tales.
To differentiate I'd use the G5 present Earth to set up the JSA as simply being X-years before the present. So the Post-Crisis "original" JSA might be from the 1990's but a decade from now they'd have debuted at the dawn of the 21st Century.