Switching back to previous employer with I-140 approval !!!

[prasen1]

I have I-140 approved with my previous employer with a priority date in 2011 in EB3 category. Within 20 days of my termination with my previous employer I joined my new employer (2017 March) and they started my GC application under EB2 category. Currently my GC application with new employer is at 3 months into recruitment period (which is after prevailing wage request completion). The attorney working on my case said they will file PERM in 2 months or so. And it will take another 4-6 months for PERM labor certification to finish. I would like to know if I go back to my previous employer, can I use the existing I-140 approval with my previous employer as they confirmed they did not revoke it. And if I go back to my previous employer can I change my application to EB2 category under the same I-140. With so much activity with immigration happening I am afraid I won’t be able to retain my priority date (2011) as my new GC application will take another 8-10+ months to file I-140. I do not like the new city and my previous employer offered me the same position. I would like to know if going back to my previous employer help me be in a safe position for my GC application. or does my previous employer need to repeat the entire GC process again?

thanks for your help!