guy betsy is correct
the OW RTW Explorer (or Continent) fare permits travel in A class in absence of a business class cabin - even after 1 SEP as per both BA and QF.
However the Global Explorer RTW doesnt allow this "free" upgrade. This fare is apparently available only in certain markets and also only in certain classes depending on the origin.
this info is from BA so take it for what its worth
and rudi - regarding *A downgrades on specific sectors - a possible way around this is to use AC or MX both of which offer a business class cabin

and these days it looks like a good idea to avoid UA anyways!