Originally Posted by
henkybaby
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.
That refers to paid sector upgrades (other than the upgrade from economy to premium economy specifically referred to in the rules).
Subject to availability, having the u/g currency, etc you can use vouchers/miles to u/g individual sectors on a RTW. I, and many others, do this.