Can I get new H-1B extension based on previously approved I-140?

[iwanttoknow]

I have an approved I-140 from Employer A, based on which I got a 3-year H-1B extension till 2019 at Employer B (current employer). Employer B has not started my PERM yet. If I change jobs to Employer C now, can I get a new H-1B till 2019 (irrespective of whether Employer A revokes I-140 or not), since my current H-1B is already valid till 2019? In case Employer A does revoke the I-140, how can I change jobs to new employer C without getting a new I-140 approved from Employer B? I'm trying to understand if having an active I-140 is necessary or can I still change to a new employer by getting a new H-1B (who'll start my PERM between now and 2019)? Has anyone faced this situation before?