Looking through the rules, the only reference I can find is under the reservations paragraph:

. ECONOMY BUSINESS FIRST
AA/AY/BA/CX - INTL L D A
IB/QF/EI/LA - INTL L D A
AY/EI/LA/LP - DOMESTIC L Y Y
BA WORLD TRAVELLER PLUS - T CLASS - ECONOMY CLS

So I guess that's the way the downgrades go, but I'm sure there's someone out there that can talk from personal experience.