H1 extension after 6years even though I-140 approved with old employer

[Indian789]

Details:

1. I got approved I-140 having priority date as May-09-2013 with old employer A 
2. I worked for the same employer more than 3 years after I-140 approved 
3. I completed 6 years on H1 visa 
4. I got approved H1 visa through employer B from Nov-1-2017 to Nov 1-2018 

Questions: 
1. Can i use "approved I-140" to extend my H1 even though i changed the employer to B? 
2. If I am not processing my GC with new employer B can I get H1 extended until priority date becomes current? 
3. To get H1 extension after my 6 years of H1, we can use old approved I-140 regardless of from which employer we are working? 
4. I need to re do the GC process whenever I wish to do the GC process regardless of employer but I can retain the OLD PRIORITY DATE?