title: Migration to change default value for a field and change all existing record's value to new default value only if it has old default value.
body: <p>I need to change the default value of a field from 0 to 3, but the catch is i have thousands of records already and want those records to change the value to 3 from 0 only if the record has default value 0 but for other values like 1, 2 it should remain the same. how can i do it? </p>