Thank you for your question.
It can be difficult to process a new PERM labor certification through the same employer that already has an approved PERM for you. First, if different requirements are being listed to qualify under EB-2, the permanent position offer would need to be different than before. Second, in general, you could not use any experience gained with the company to show you meet the requirements of the new permanent offer unless you proceeded under the exception to the rule and showed the previous/current job was at least 50% different than permanent job offer. Third, even if you are able to proceed, DOL could audit and request that you withdraw the current aprpoved PERM in order to proceed with the adjduciation of new PERM filing. If the new PERM filing were denied, you would be left with no green card process in place.
In order to proceed with new EB-2 process with the same employer, a new PERM and I-140 would need to be filed. That process and pros/cons would need to be discussed with your employer.
Whether a new process can qualify under EB-2 depends on the position offered and its requirements, your total education and experience, DOL's view on normal requirements for the position offered etc. That would need to be evaluated on a case by case basis looking at many factors.
If you are looking to join a new company and start a new green card process through them, and initially if you are applying for H-1B extension beyond 6 years under AC21 through the new Employer, it may be prudent to obtain the new H-1B approval through the new company before leaving your current company. One risk in general is that the new H-1B through the new company is denied and you already left your current company. Another risk is that your current company withdraws your approved I-140 before the H-1B through the new company is approved, and that could affect your H-1B extension eligibility under AC21. You therefore may want to consider premium processing for the new H-1B filing.
If you have not been able to file 485 paperwork because your priority date is not yet current, you cannot take your current green card paperwork with you to the new company under AC21 and file 485 portability paperwork. You will have to start a new