How to amend I-140? Please help.

After several year waiting, I realized that my employer submitted my I-140 in a wrong category. I am supposed to be EB2, but it was accidently submitted as EB3. The attorney hired by the University agreed to amend I-140 for me and keep my priority date.

Does anybody have the same experience?

How long will it take?

Is there any bad consequence?

Thank you very much!