This is the answer from my company's lawyer in NYC (Fragomen, Del Rey,
Bernsen & Loewy, LLP)
Question:If the EB3 is declined, does Weiyi pop back into her position and
standing in the EB2 category or do we start over? We don’t want to lose
time.
Answer:Exactly, she will just continue on waiting for her EB-2 priority date
to become current and then file her adjustment of status application at
that time. Filing for the "downgrade" is not done at the expense of her EB-
2, that remains available to her if the EB-3 is unsuccessful.
just FYI to all of you. 不好意思, 我当时没有问 EB-2 和EB-3 的 I-140可否并存.