I have a model that I store in 'localstorage'. The model started out with 4 fields (call it version 1) but over the course of development, I added a couple more fields to it (version 2). I now need to code for the situation where version 2 code encounters version 1 records in 'localstorage'. I refer to this as data migration. What is the best practice to handle this situation in Sencha Touch 2?