H4 to H1B Change of status approved and employed with another employer

[ShikhaKaks]

I have applied for h4 to h1b with change of status with employer A. I received my approval notification today but I haven’t received approval notice yet from USCIS. 
I am currently working with employer B on my h4 EAD.
1. Do I need to resign my job with employer B immediately?
2. Can I wait till I receive H1 approval notice?
3. Can I continue my employment with employer B till I start working  with employer A i.e. till I-9 verification starts with employer A?

[LeaveMessageH1B]

1. No

2. You cant do without it. 

3. Yes, this is obvious.