gosh - what a mess! I once had to write them to get original XP credit for a downgrade (W to Y) on a DL flight that had an aircraft change. I finally got the original XPs for W, but they somehow reduced the miles that were earned (W should always earn more miles than Y?)
It's likely that when you were downgraded, the "fare paid" information got lost, so you earned deistance-based mileage (and likely a high coefficient, since you probably ended up in a high RBD).
Going for original booking credit required the agent to look up the original ticket, which included the fare information.
Depending on the design of the system used, if it works by "use this ticket info to override", agent might not even have any control over mileage posted.