There are a number of bugs on file which imply that for some users, Sync never completes successfully due to one or more engines either not completing, or continually succeeding with one or more failures. Part of the migration plan will probably involve doing a full sync of the "old" account before doing the migration - however, we need some plan on what to do for users where this doesn't happen. Will probably involve additional UX (eg, we might just show them some UI but offer to go ahead with the migration anyway)

Our current thinking is that a fully sync is not necessary to perform migration. A sync of the clients engine is, but that is far less likely to never complete.