According to Delta, since Covid there are no longer fees to cancel or rebook at a lower price, We have 9 tickets to Europe this summer on Delta and when I saw a price drop ($350 each) I gave them a call and they were as nice as can be.
Each passenger was given a credit in their name and it needs to be used 1 year from the date of the original purchase.
Not sure about other airlines but the agent made it sound like it was for all airlines.