As a data point for this on another airline (United) this happened last summer and this was the resolution

We had IAD-BRU-ARN, then HEL-FRA-FCO then FCO-IAD booked. I found a better option and called united and asked them to change to IAD-BRU-HEL, ARN-FRA-FCO, FCO-IAD. The agent saw the tickets and did whatever process they do. THey came back and said well we changed one flight but the second wouldnt change for reasons? So she is like i have one confirmed and the other is the original. I was like well that wont work obviously, just change it the original and we can look at it later. She goes to do that.. and she had deleted the original from the first PNR and couldn't get it back. Long story short i found a TLL-FRA-Rome that worked (and got us a bonus country since TLL is a 2 hour ferry from HEL and we probably were going to go anyway) and they were able to ticket that.

We asked for compensation and they gave us 10k miles each (including to the lap infant, her first miles!) and they gave us 3 $125 vouchers as well. Something like that or more seems appropriate.

On a side note i wouldn't skip a graduation just because i have to fly economy. I mean really?