Change of job title with the same employer with approved I_140

[GC_2011]

Hello,

My current I-140 is approved 6 years back for the role of “Technology Manager”. Recently I got promoted to the role of “Product Manager” by my employer. I understand that under the AC21 I can keep the PD. I was advised that my GC will have to start all over again and I need to get my H1B amended. I am in the 10th year of my H1B. I have a few questions listed below and I appreciate your advice.

a) Should I file the H1B amendment first or start the the new PERM first? 

b) I have already started working on the new job. Is it OK to start now or does my employer need to change my position back to the old role. 

c) The new role is only 40% different from the old role. How should I prove that? Do I still need a new PERM and H1B amendment?

For reference I have two graduate degrees (PhD and MBA) from the US.