Yes, you can migrate from SQLite to ForestDB using an explicit API call (Manager.openDatabase, setting the storage type to ForestDB in the DatabaseOptions parameter.)
I can’t comment on the PhoneGap plugin since I don’t work on that; but I agree that we should add support for ForestDB to it. Have you filed an issue on Github?
I haven’t created a Github issue yet, I will do that.
I am not yet using Couchbase Lite in production, but am going to use it with Cordova. I would rather not start modifying the Couchbase-Lite-PhoneGap-Plugin now, but since migrations are supported, it seems I don’t have to worry about switching over to ForestDB later on.