BaseCamp Classic let's you export projects into XML files. This XML file covers all of your to-do lists and to-dos.
Click the + icon in the top left corner of your collection and choose Import instead of Create. This will open a dialog where you can choose BaseCamp Classic and then browse for your exported XML file.
When you've chosen the XML file you wish to import you will have to match the exported BaseCamp Classic users with users in your Favro organization.
Once the import is finished there will be separate backlogs created for each to-do-list and cards with comments created from your imported to-dos.