These have probably been asked and answered many times, but its a long thread:

1. I book a trip on the 10th March 2020 for $900 for a trip in June and now want to change it, new trip costs me $850. If I change the trip, then I presume I am out on $50. If I cancel the original booking and get future flight credit, would I have $900 in travel funds, use $850 for new trip and then have $50 left over to apply to a new trip?

2. Unrelated to the above. I book a trip in economy on the 10th March 2020 for $900 for a trip in June and upgrade 1 of the 4 legs after purchase to business for $189 (the offer after booking, which books into P). I then cancel this trip. How much in future flight credit do I now have?

Is it always, with this waiver, much better to cancel a booking, rather than change it?