From the AA site: A oneworld explorer ticket cannot be partially upgraded to a higher class. The whole ticket must be upgraded from the point of origin.

And I believe this goes for all airlines. Don't think that MFU is an exception.

EDIT

And I maybe wrong.

Here is the answer: http://www.flyertalk.com/forum/3024167-post2.html

Although BA has no on board upgrades anymore and the whole thing maybe out of date. It is worth checking before you book.