This is a popular misconception. You CAN MFU any ticket (rules and availability permitted) but the upgrade has to be processed by the issuing country.

If, for example, you do an ex-MUC and want to subsequently MFU it then Bremen, the local office, has to do this.

Similarly if you are doing an ex-USA and want to MFU this then you'll have to call the US EC to process it.

You can get around this by booking the MFU online at the time of booking because everything is processed and ticketed at once - although BA's systems often collapse mid-process so I can understand why people don't do this.

Note that you will be billed in local currency so need to take that into account when paying for it.

BAH