If I were to speculate, I would think the "neo-relic" quest line would have a progress bar, similar to how lights are accumulated. New players can begin building their "neo-relic" at this stage and get a weapon.

Veteran players would be able to trade in their Relic / Zodiac to "evolve" into the "neo-relic". The further along you are in the original quest line, the more progress you accumulate in a single trade, with a Zeta being worth 100% of the progress (therefore instantly evolving)

This seems like one of the best ways to maintain the existing relic quest line without having new players grind out the entire chain from scratch. Also the veterans would get the bonus of having a weapon with "customized" stats versus a new player with stats provided by S-E.