H1B Transfer to another employer

[sm2016]

Hello Everyone,

Background :

My current employer filed I-140 petition which was was approved in Nov 18, current project with our client ends  April 19 and so is my H1B extension, my 6 year completes in Sep 2020, therefore, would like to know if I switch employers and file an H1B transfer what are the implications :

Questions :

If I switch to employer B and file an H1B transfer would that invalidate the approved I-140 petition and prohibit me from applying for the 3 yr extension after completing the 6 years?

After making the transfer to employer B, can I File extension based on approved I-140 petition from employer A?

Would the priority date become invalid and require me to file new PERM and I-140 with employer B for receiving the 7-year extension? 

Thank you in advance for your time and consideration.