I have L1-B visa with my current employer. It expires soon. But I just have got approved my H1-B. Also I have my I-140 approved but stuck in EB3 row. I want to change employer but want to know the following:

1) Do I need to wait until my H1-B be effective to apply for EAD?
2)What will be the EAD period that allows me to work?
3) How long can I stay in the US with my H1-B now? I understood that I can extend my H1-B until get my GC approved, but how this could effect if I changed employer?
4) If I decided to leave my current employer, I will loose my GC process and I-140 too, so I will have to start all over again with other employer?
5) If you keep your I-140 approved for more than 180 days, then you earn some right to follow GC process w/ other employer?