Originally Posted by
G-BOAC
No need to assume, it's a fact. And all very well documented in the sticky here and on ba.com. MFU from economy - shorthaul or longhaul - from YBH fares only.
I have had situations where it costs less to MFU from WT+ to Club World than to MFU from WT to WT+.... seriously... as you can MFU any WT+ fare but only flexible WT fares. This makes CE MFUs rarely worth it..