After 30 mins on the phone with a Concierge today, I learned that you can't easily refund to a wallet if you book a fare with mixed fare brands.

I had a flight that was Comfort brand to Europe, returning in Flex. I wanted to make changes and also upfare to PY. I couldn't do this to both outbound and return online (error messages), so called in. For some reason, the mixed booking class made it difficult to change both out/return (change price was coming back at more than just buying a new additional ticket in PY flexible). They offered to refund to wallet so I could just buy the new ticket, but found out that they couldn't refund to wallet with a mixed brand booking. After a long time on hold while they talked with a supervisor to try and make it work, we gave up and I decided to just change the return online.

Happily, just a minute or two later, the supervisor called me back as they'd found an over-ride to force a refund to wallet, but it seems far from easy.