Well if you are removing fields from that json, then the new model would just omit those fields and ignore them, no harm no foul, and the new/updated JSON that gets inserted later on into your table just wouldn't contain those fields.
If you add new fields, you could specify default values or mark the new field as nullable, which would safely handle converting old json stored in your table that doesn't contain the new json fields.
If you wanted to migrate the value of a field that is present in the old model and in the new model, simplify load the record, modify the field in the json string, reinsert the record.
Hope I understood your question and this was helpful..