Another extremely important caveat is that you can only upgrade a ticket that has been purchased directly from BA. This applies to ba.com or over the ‘phone. If you are in North America, you can upgrade a ticket, but it must have been purchased from a “BA Preferred Travel Agent”.
One-way MFUs are possible at any time for half the miles (For posterity, previously you could only do this if only one segment had availability for the upgrade and the other didn’t – this could often require some lengthy discussion and quoting ‘AskBA 1056’ to the agent!)