The whole idea behind migrations is that each change you make goes into a new version. To add a new field, create a new migration file:
script/generate migration adding_new_field
and then add edit that file and add
add_column :table_name, :new_field_name
for each of the new fields that you have to the self.up method. Remember to add
remove_column :table_name, :new_field_name
to the self.down method for each of the new field.