Switching employer after I140 approval?

I am working for a small desi consulting firm and completed 6 years in Jan, 2017 and I140 is approved in Feb, 2017 and H1B is good until Dec, 2017. I am working as a Consultant for a decent size good company and they are asking for FT conversion.

  1. Do I need to wait 6 months to switch employer to retain priority date?

  2. Can my ex-employer rovoke I140?

  3. In how many days then new employer needs to start Perm process?

  4. Can I get unlimited 3 year H1B extensions based on the I140 filed by ex-employer?