What is the best upgrade path for dirstate-with-subtree?

[Kevin Light]

I have a few repositories that would like to be upgraded since moving to 1.6.  These repositories were moved to bzr from subversion and bzr info shows a format of dirstate-with-subtree.  I have not used the subtree features.

I've seen it mentioned that rich-root-pack is a solution but is that the best?  Doing a bzr upgrade always fails when using the default format.  What is the recommended format?  

Kevin