Please help! How to transfer I-485 application from EB3 to EB2

I know this question might have been asked before. I searched around but could not get a straight answer. I hope that some one would help to answer it again.

I have a EB-3 I-140 approved first and my current I-485 is filed based on the EB-3 I-140 approval.

While the I-485 is pending, I filed a EB-2 I140 and it was approved lately.

My question is, to enjoy the earlier PD of the EB-3 application, I would like to transfer the pending I-485 from EB-3 based to EB-2 based. To make that happen, what do I need to do? ( By the way, my attorny is not helpful. )