The solution (I think) is simple but requires some manual work. CB Subs will add any user based on a delimited file. So new users can be added by exporting the list from the old DB using CB Juice and then the file can be winnowed to only include those users who are new since the migration. That's step one. CB Subs has the ability to mass update any user to any plan by using their IDs. You can also supposedly add users directly into a plan by creating a delimited text file with:
This text file is multiple lines (one line per user) comma-separated list with following format: userid,planid,statusSingleChar,SubscriptionDateISO.
Status: A = Active, X = Expired, C= Unsubscribed/Cancelled.
Example: 63,1,A,2009-01-01 00:00:00
I haven't tried this but I'm sure it would work and it may be easier than the two step process I am describing but I had some inconsistencies with keying the Membership plan. I will try it both ways at some point.
So, what I know works is: 1) add any new users by using CB Juice and 2) upgrade any current and/or added users directly by using their ID in CB Subs > CB Paid Subscriptions > New Import subscriptions.
Or -
Just try to create a text delimited file to do both at one time.
Mark