In an attempt to answer your questions, social workers change because they leave, work changes, a case is passed to a more senior social worker or sometimes relationships break down.
I'm still desperately trying to understand your issue but you're really not very clear; my understanding so far is that there was a malicious call in August which you knew about and this was removed but in December 2013 you heard about a referral in July which you knew nothing about and this is where you are stuck. It could be an oversight or an error, you can ask about it but to be honest with you'll just be getting yourself bogged down in something silly when I'm guessing the real issue is that of the domestic violence and the emotional abuse your children/child may have been subjected to as a result of this; assuming that this has been going on longer than the referrals to social services then I would say that the July referral is a complete red herring to you and you are best placed engaging with services fully, telling them everything and working with them to safeguard yourself and your children.