I have run into this problem repeatedly, and it is very frustrating. Someone on another thread once pointed out to me that in cases where you had an involuntary schedule change on the itinerary, you will not be able to make changes online until you “acknowledge”/“accept” the change on DL.com.
I haven’t tried this personally, so I can’t verify that this is indeed the case and YMMV, but it may be worth trying clicking the acknowledge/accept button on the trip page if there is an unacknowledged change and then trying to change online.
If this works for you, please report back—I’m interested to know!