Originally Posted by sdorling
The simplist way is to MFU via the using my BA miles link in the executive club tab - you will of couse need to be logged in - and then click on 'upgrade a new booking with BA miles', once there it's all self-explanitory and you any transaction will ensure you are booked into an eligable fare bucket.
If you don't have the miles yet, but want to buy a ticket to upgrade at a later date, you can either call the exec club and pay the £15 offline fee and they will be able to ensure you are booked into Y,B or H or, just booked a flexible economy ticket as i believe Y,B,H are all (semi)-flexible so you should get an eligable fare that way.
Alternatively check at the end of the process , if it states you will be invited to join the exec club, this means you will be earning tier points so have an eligible fare.
I would do it via the exec club link as stated by sdorling as then you'll be sure its done properly.
Chris