Domain holders who choose to migrate to ENSv2, the upgraded ENS naming protocol, undergo a permanent transfer where original v1 tokens move to an un-extractable Graveyard contract and cannot return to the old system

From the articleEthereum Name Service plans major .eth overhaul with one irreversible catch for domain holdersAlso in Ethereum Name Service Team Asks DAO to Switch On ENSv2 and Freeze New .eth Sign-Ups on v1 - Unchained, ENS seeks approval for major .eth domain upgrade