Hi TG,
B,C and D are reference of A. so you no need to delete data from A. you can add required attribute A IO and save, activate. Load data data and run ACR. everything will be all right.
Why you won't delete data, if you go for deletion, first you need to delete transactional data where this A,B,C and D objects are used. business won't allow you to delete.
Without delete you can add required attribute IO - A, it will reflect added attribute to B,C and D.
Thanks