The online tool to convert your bmi Diamond Club Miles is now LIVE at www.britishairways.com.
Go into your account, Click manage my account and then Combine my Avios.
A FULL GUIDE on how to convert your miles to Avios is published on GhettoIFE.com at http://economyclassandbeyond.boardingarea.com/2012/07/02/a-guide-to-migrating-your-bmi-diamond-club-to-british-airways-avios/
