Strictly speaking, by the 1970s they had already acknowledged 1955 as the cutoff between Earth 2 and Earth 1 Supermen and they were considered 2 different characters. And starting with Crisis on Infinite Earths, DC rebooted every other week almost.You don't have to do a reboot every time you change the sheets on the bed. Superman existed from 1938 to 1986 without a reboot. The creators made adjustments along the way adding and subtracting elements as needed for various reasons. Superman's WWII adventures just stop being mentioned as real time moved away from the 1940s. Superboy was inserted into continuity in late 1940s. His costume underwent changes as did his Kryptonian name. Minor tweeks were made like the Daily Planet and Perry White.
Marvel has a better track record for non-reboots in the comics, except for that bit where Spider-Man's marriage to Mary Jane was erased. Everything that happened in Marvel comics, even the 1940s Captain America comics, still applies to the current version of the comic characters.