Starting over for the expansion seems like the most logical thing to do. I know there was a lot of people that were upset at the system when it first came out. People did not like how awful the drop rates for atmas were. And I imagine there are still people like this. Starting over gives developers and players a chance to has a new chance at this.

In addition if they were to require the previous upgrade relic to start the expansion version, it seems like they would need to also create either a kind of catch up series of quests or an entire quest that essentially lets players bypass the entire previous relic upgrades just so they can then start on the new relic. It seems like this option would be more hassle than just having everyone start fresh at 60.