H1B expired, I-140 approved, Out of the US, Can I change employer?

[bs2018]

Following is my current situation:
- My H1B visa expired in Feb 2017
- I am currently outside of the US since Jan 2017
- But I haven't completely used 6 years on H1B with approximately 30-40 days of time that can still be recaptured
- My I-140 recently got approved
- Due to some reasons, I am thinking of changing the employer

I have following questions:
- Can I change the employer and the new employer file for H1B transfer + extension even while I am outside of the US?
- Can I then directly travel to the US on the H1B visa sponsored by the new employer?
- What about I-140? How soon should the new employer start my GC processing?

Thanks!