I just looked at my most recent ECDSA certificates, and I don't see any alternate chain option as of now.
I also do not expect that: Most Let's Encrypt engineers are somewhere in between Pacific and Eastern time, so even if the deployment happens today it will take some more hours. I would advise to just be patient and look for an announcment in the appropiate thread: Root X2 alternate chain for ECDSA opt-in accounts
In staging the alternate chain is already available, as mentioned by @MikeMcQ. If you want to experiment now, do it there.