I think it would be a matter of substituting one port for another, rather than replacing a sea day with another port. The eastern islands are further away, and our cruise was already very rocky and windy (so windy that the Aqua Duck was closed on 2 sea days) as we raced to get there over the course of the initial two sea days. I can't imagine trying to get to the Eastern islands in just one day. And later, it does take them a full day to get from the Eastern islands up to
Castaway Cay.
We're booked for the 9-night Eastern in 2020. That one will have some ports of call (St. Kitts, Martinique, Antigua) that aren't the usual for their Eastern cruise. But I also wish they would mix up their standard cruise itineraries more.