I wonder how many people owned an (i) model of the same headphone…and got an (e) model to replace them for what ever reason, and didn't/couldn't tell the difference until they were told they have an (e) model. Speaking about the earlier (e) models of course, without an (e) on them.
Legally speaking, most every company "reserves the right" to deliver an equivalent value or demonstrably superior product if a particular item is out of stock or being discontinued.
Practically speaking, I find it very difficult to believe that the e series will be inferior in durability or quality. Based on what I've read and the few anecdotal reports trickling in, the e series features significant upgrades across the line and sounds a bit better than the i series.
Grado doesn't revise it's line very often, and given the list of changes, the e revision is a near comprehensive overhaul.