I don't know if Eb3 to EB2 is at all possible. If possible I am sure the following criteria would have to be met:
1. Original labor must satisfy EB2 requirements, i.e. the wage and qualification requirements mentioned in the LC must satisfy EB2 requirements.
------------one can file new LC in EB2 and once I-140 gets approved under EB2 one can interfile the newI-140 with pending I-485
2. The old I-140 must not have been revoked.
----------if employer revoked it is not a problem, if USCIS revoked under sections 204(e) or 205 of the Act then problem
3. You cannot "convert" existing I-485 to EB2. A new one must be filed.
------------wrong, one can interfile second I-140 approval notice copy with pending I-485
Also, I don't think you can file a new EB2 labor and use the old EB3 labor's PD.
-----one CAN
The two applications are different.
--------- not a problem both are Employment based petitions, EB2 or EB3 does not matter.
Retention of section 203(b)(1), (2), or (3) priority date. -- A petition approved on behalf of an alien under sections 203(b)(1), (2), or (3) of the Act accords the alien the priority date of the approved petition for any subsequently filed petition for any classification under sections 203(b)(1), (2), or (3) of the Act for which the alien may qualify. In the event that the alien is the beneficiary of multiple petitions under sections 203(b)(1), (2), or (3) of the Act, the alien shall be entitled to the earliest priority date. A petition revoked under sections 204(e) or 205 of the Act will not confer a priority date, nor will any priority date be established as a result of a denied petition. A priority date is not transferable to another alien.
*** read/search for PEARSON MEMO of May 9,2000