Applying from EB3 to EB2

My question is re. applying in EB2 after applying in EB3.
I have heard of a couple of cases where -
Labor certification was not required for the EB2 case. Instead
the LC of EB3 was used for the EB2 case. As a result the applicant could file directly for an I140.

Can anyone please tell me -
1. Is this possible?
2. Under what conditions is this possible?
Thanks and best wishes.