If your employer does not allow you to upgrade your petition, you have nothing else to do. Company did a big favor in allowing you to downgrade. When you applied for GC, you were qualified with skills required for EB2 and it excluded lot of candidates who would otherwise have qualified for the job in EB3.
I had presented this possible scenario early this year, but few members felt I was disrespectful and inconsiderate to the people who wanted to downgrade and their chance for getting an EAD. EAD without approved 140 is basically an useless document.
I think you should have patience and see when your Eb3 140 downgrade is approved.