Green Card: Going back to previous employer

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I worked for employer A on H1-B visa (valid thru 04/30/2018). Employer A had also started my GC process with approved I-140 (EB-2) with the priority date of 10/07/2009. I left employer A and took employment with employer B on 07/10/2017. Currently, I am employed with employer B. Employer B transferred my H1-B with start date of 07/10/2017. My current H1-B with employer B expires 07/09/2020. Employer A informed USCIS about my leaving the firm and canceled the H1-B visa. 

Now I want to go back to employer A starting January 2018 on the same exact position and job description. Employer A will file an application for H1-B visa transfer with premium processing. How will the GC process work in this case? Can employer A file I-485 when the priority date re-opens again or would employer A have to start the GC process again?