Was this retconning necessary?
I would not call it a retconning.
What we see in the original gantz is something that happened when the gantz ball wasn't sent in the past.
Basically we have the original gantz timeline, somewhere a gantz ball with time travel abilities get sent into the past. And then from this future past the EDO gantz are sent into the future future that is after the events of gantz.
Yeah, it's kind of confusing.
TL;DR:
Take it as parallel universes or different timelines. Nothing from the original gantz get invalidated. It all happened in a different future past.