If I485 gets denied, what happens to Priority date, H1B extension

[Alpesh shah]

Hi,

My priority date is current. My I140 was approved in 2015 and I am ready to file I 485. My current H1B is valid till Sep 2021. I have a few questions

1) What happens if I485 gets denied. Do you lose your priority date and have to start over from zero again?

2) with H1B status till next year, I am aware that I can stay and work till sep 2021. How about H1B extension after that. Is it possible to extend H1B from past I140? Do I have to file new I 140? 

3) How much time would I have to figure all this out, since if my I485 gets denied, most likely it would be June or July.

4) If I485 gets approved, do I get green card right away or is there a waiting time in that too?

5) Can I downgrade from Eb2 to Eb3?

Thank you.