Thank you for providing the most recent email response from e-Rewards.
It's indeed a sad news for those who joined e-Rewards using Delta's invites. On the other hand, the new quarterly Skymiles redemption policy will also allow up to 8K miles per year (if enough e-Rewards currencies are earned, of course). From this angle, the new change is much better, if Skymiles are the chosen redemption options.
I don't think I enrolled thru any airline partner and I am not affected by this new change. When I log in to my e-Rewards account, not only can I still see all the airlines as my redemption options but also Skymiles appear to be a quarterly item. All the other airline redemptions remain as annually.
It's not clear from the official reply email (Iphinome kindly provided) whether non-Delta-sponsored accounts can do quarterly redemptions for Skymiles. The email seems to imply quarterly terms only good for Delta-sponsored accounts. I usually don't go for Skymiles so I won't be able to test it out. It would be nice to hear from someone in the know.