This is incorrect for an EB2-EB3 downward porting scenario.
In a downward porting scenario, the EB3 I-140 is being applied for on the basis of a PERM that has already supported an EB2 I-140.
The EB3 I-140 is using the same PERM, which sets out both the minimum requirements and minimum wage for the position.
Regardless of whether an EB2 or EB3, or both I-140 are applied for, the I-140 application needs to meet the minimum requirements laid out in the PERM certification that supports it (Educational, Experience and Wage).
An EB3 I-140 would have to show a wage that was the higher of either the Prevailing Wage or the Offered Wage in the PERM.
The scenario you mention would happen only if a PERM was certified which did not meet EB2 requirements at the I-140 stage. Then it could have lower requirements and a lower Prevailing Wage.
By definition it would not be an EB2-EB3 downgrade. It's just an EB3 application.
It might also be considered fraudulent, if the employer was expecting the beneficiary to carry out the (unmentioned in the PERM) tasks associated with an EB2 application for a lower wage. Lower requirements might also cause it to fail at the recruitment stage, since more qualified applicants might apply for the position - a position probably inaccurately described.
There's no maximum requirements for applying under the EB3 category at the I-140 stage. Whatever category is used at the I-140 stage, the PERM must be an accurate description of the job offered and the minimum requirement for that job.
Even in a EB3-EB2 porting scenario, if ultimately, the I-485 is approved under EB2, the Prevailing Wage from that PERM and I-140 would be applicable.
Only if the beneficiary interfiled back to the EB3 and was approved under EB3, then theoretically, the Prevailing Wage from that PERM could be said to apply (but only for the position and duties in the EB3 I-140). Only an unscrupulous employer would use that, because the beneficiary would (presumably) actually be working at the EB2 level. In that case, they could argue the EB2 PW applies, or leave the employer.