NEVER change award flights using the app...
Yes, should have known better...
I had booked a roundtrip economy ticket for a total of 60,000 miles (30,000 each way).
Decided to use the app to upgrade the outbound to J for a total of 90,000 miles (60,000 outbound, 30,000 return).
Ticket re-issued. I was billed 90,000 miles. Now I have been billed an additional 60,000 miles. And obviously MP support is no use...
Moral of the story: ALWAYS just cancel your ticket yourself. Then buy the new one...