Mr Blond, I don't know a great deal about the pharmacology of buprenorphine and i can only go off personal experience and from what i've read. If someone was dependant on full agonist opioids while not in WD then it's possible it can lead to a precipitated withdrawal.
this is from wikipedia; People switching from other opiates should wait until mild to moderate withdrawal symptoms are encountered. Failure to do so can lead to the rapid onset of intense withdrawal symptoms, known as precipitated withdrawal.[25] For short acting opioids such as codeine, hydrocodone, oxycodone (and OxyContin), hydromorphone, pethidine, heroin, and morphine, 12-24 hours from the last dose is generally sufficient. For longer acting opioids such as fentanyl and methadone, 2-3 days from the last dose is needed to prevent precipitated withdrawal.
So you're currently using 1200mg of codeine a day? 600mg twice a day i assume... well i'm not exactly sure how you'd go switching, but i assume it would be ok due to codeines short half life. I can't remember what i actually read about tramadol working with buprenorphine but they seem to work well and not cause any unpleasantness when switching. like i said, i used tramadol for 24 hours then switched to bupe with no WD effects what so ever.
now regarding the dose; thats something you need to work out with your GP/specialist, but i would suggest starting at 4mg then reducing 0.2mg each day for 20 days until you hit 0.2mg... that way you will be at home, comfortable and you wont even think about using codeine in that time.... beats doing an in house detox.... anyway, let me know how you go, whatever you decide to do.