If I understand this thread from a couple months ago correctly, they're still working on the back-end changes needed in order to issue ECDSA certificates from a different intermediate than RSA certificates. At this point they haven't announced a date or when E1 will enter service, and the certificates page just has it as "upcoming".
The January 11 switch in the chain being served by default isn't related. I would hope that it would happen before September (I mean, that'd be a year after they generated E1) but that's just me being optimistic.
Are you currently using ECDSA certificates, but need to use the IdentTrust root until September? If so, I suppose if you switch to RSA than you could be sure that Let's Encrypt wouldn't change to signing from an intermediate that can't chain to IdentTrust before then. Or, just keep on eye on the API Announcements category and you'll probably get plenty of notice before E1 enters service.