As I learned today after spending 20 minutes on the phone, buried deep in the Easyjet T&C is the following:
9.2.2 If You buy an easyJet Flexi You will be able to make an unlimited number of free changes to the Flight time in Your original Booking, subject to the changed Flight time falling within a fixed 4 week time window (starting 1 week before the original travel date and ending 3 weeks after the original travel date). Changes to your easyJet Flexi Booking are subject to seat availability on alternative Flights. easyJet Flexi Bookings do not allow route changes (i.e. You are not entitled to change the departure or destination airports of Your original Booking). You must make any change to Your easyJet Flexi Booking at least two hours before the departure time of Your original Booking, otherwise You will forfeit Your ability to make a change.
Therefore, even though I purchased a more expensive fare, to gain the flexibility of date changes, I ended up holding an even more restrictive ticket than I had intended, since I was completely unable to change the ticket routing (even for a fee).
In the end, I'm not out of pocket anything, as this is just going to get passed along to my client since they were the ones who caused the change in schedule, but I did want to warn others that it appears that you're almost always better off booking a traditional fare and then paying the change fee if you need to change your ticket.