Hi Abira,
If you add new columns to a table with records, these records will take the default value of that data type (false for booleans, 0 for numbers, '' (empty) for text and 1900-01-01 for dates).
What is usually done if you need to "fix" these columns, is to have a timer that updates these columns depending on your business case, properly tested in previous environments. The new records will take the new logic of the code and they will be ok. This timer is then of course removed on Development environment once it ran successfully on the code so you keep your code "clean".
Regards,
João