請教aftercard, 小白兔or其他高人, my questions post again

Sorry, I have to post my questions again. I wasn't clear the first time, so aftercard misread my questions.

I filed NIW but is still waiting for EB2 priority date, so I can't apply for EAD. Can I change my status from H1 to H4? What if it's only for a short period, and I will find job in similar area soon. Would the gap affect my 485 submission and subsequent approval?

Second question, my H1b at my current employer is approaching the 3 year limit on July 26, 2007. I am not sure if I want to file extension. What is the latest time I need to file the H1b extension, if my employer choose to do so? Do I have to get it approved before my expiration date, or just the receipt notice will suffice?

Thanks a lot!