H4 to F1 COS applied. GC Priority Date become current ?

[rgupta01]

Hi, I am on H4 and got an admission in a university for MBA. I have already applied for COS from H4 to F1 and its pending at California Processing Center. My husband who is on H1B and his I-140 already cleared. We think his priority date will be current next month. I have following questions

1) If the priority date becomes current and my F1 is not approved can I withdraw the COS application and apply for I-485 ?

2) How much time does it take to withdraw COS to F1 ?

3) If F1 is approved then can I immediately apply for I-485 or do I need to wait for sometime ?

4) If F1 is approved then do I need to get it stamped before I apply for I-485 ?