There's a few ways to do it, but you will have to go through two stages of regression regardless of the foundation size you use.
1: Shake them down to 4.9 foundation
2: insert frames into the broodnest and rotate the large cell frames to the outside. Keep doing this until the center of the broodnest is mostly 4.9
You can also use 5.1 foundation. That's what they will initially draw anyway, and then you can feed in the 4.9
I would highly suggest though that you read both Dee Lusby's articles, linked on Michael Bush's website www.bushfarms.com
and join the organicbeekeepers group on yahoo groups. Both are good sources of information on regression and running small cell.
I am in the process of regressing a hive right now by feeding in frames of starter strips.