COS H4 to F1-Deferring admission

Hello -My COS H4to F1 is pending and now I may have to defer admission to next semester because the school is starting and did not hear anything back from USCIS yet. So can i just write a letter to USCIS and say that the COS is for new dates now.
Since my H4 is expiring next month, do i need to file a separate application for extension of my current H4. My husband H1 extension just got approved and it was not filed concurrently with that.
Any help will be appreciated.