If it were my body, I would try 10-mg and wait 2 - 3hrs, then if you aren't where you'd like to be take the other 10-mg or maybe just 5-mg. It's such a long lasting and subtle high - if you take the right dose - too much and it's not fun. This is not really a great substance to get high upon, if you're switching from one to the other the drs should know what dose to give you.
All that said, people on maintenance therapy switch over from 30-mg methadone to 8-mg bupe (roughly) with no problems in most cases, when it's done properly. But... that's going from full agonist to partial, this is the opposite.
Maybe someone with experience at these exact or proportional doses could comment. Although proportional loses it's validity at doses of bupe above 16-mg imo, b/c of the antagonist/ceiling properties of bupe.
Be careful, and do not drink/benzo - even if you have on bupe this could be much different in terms of synergy. Be smart.