Changing to new employer (I140 approved + H1B is in process with current employer)

[Medi]

Hello,

I have finished my 6 years on H1 and I140 approved (2 years ago) with my current employer. I also have a H1 renewal in process with them. 

Now I got a good offer and want to join new employer. 

1. Is it safe to change employer considering H1 extension is not yet approved? OR I should put current H1 to premium processing, have it approved and then change employer.
2. Is it safe to join new employer on H1-B receipt?
3. Will my current employer get to know about my this new H1-B processing (if my future employer initiates)? I want to take experience letter after resigning them as people do run into issue during GC process without experience letter.
4. I should do this after JAN- 17-2017 so that my current employer won't be able to revoke my I140, correct?

Thanks,

Mayank