Originally Posted by
lemonpeace
I'm unclear on how they explained it, I'm mainly pulling knowledge from the top of my head, but I believe for a while the implication was that once they integrated into New Earth, their history and memory began to change over time to match up with the new reality they were in; since New Earth now had to combine the histories of characters that were once on different earths like the JSA. Zero Hour, and i believe some other stories but mainly Zero Hour, saw their history and memory fully re-adjusted to fit New Earth. so think of it like, between CoIE and Zero Hour the Earth 1 heroes who were outside of the collapse were semi-intergrated into New Earth but after Zero Hour everyone is pretty much 100% New Earth with definitive timelines and new origins etc. generally when there is a reboot everyone's memory is restarted to fit the new timeline of the new earth.
now however, after Death Metal, everyone has access to their past lives from previous reboots. how that will work practically is yet to be seen.