I was booked HEL-ARN-DUB and managed to get on the earlier HEL-CPH-DUB at HEL check-in. The person had to call support but it was sorted after a couple of minutes.
It seems your ticket is not assigned to the new flights so not possible to change seats online or using the app, but using the check-in machine I could change seats.

This is a very good benefit, though I think it should not be for diamonds but rather for all passengers holding a plus pro ticket.