I have reached 606 IL and can no longer use Carved Crests for upgrade, but did not get ability to convert to Ruined Crests. I suspect it could be related to a message database error I also received when I opened the mail containing the conversion message. Character Castigan on Staghelm.
Are you sure? I still see some pieces at < 606
Overall item level doesn’t matter. You have to reach the threshold in every gear slot at least once. Your neck and second trinket are below 606.