As mentioned in this article Trump’s Ban worked favorable to India/China backlog significantly.
Know the Green Card Process Flow – Explained
It is not recommended filing the downgrade right now, one can always file it when the EB3 priority date becomes current.
You do not have to do in advance and also it is advised to contact the employers and make sure the employers are willing to downgrade.
Downgrade from EB2 to EB3, if you have an approved I-140 under EB2 and the priority dates become current for the EB3.
You are entitled to use the old approved labor certification, one received from eb2 to apply for the eb3.
As the USCIS rule says once the labor certification is approved, you have a 180 day window to file I-140 and if I-140 is filed within that window it effectively makes the labor certification good forever.
Based on the recent history, normal processing of the I-140 would be fine, as original labor certification might have been approved a long time ago, so the immigration service might oversee and deny the premium processing and return the entire applications.
As always, once you receive the receipt notice of the I-140, one can convert to Premium processing.
As far as Cost, the filing fee for I-140 is the only additional bit for employers and no other additional fees to go through.
The priority dates often retrogress, so it’s best to act fast. You can use approved I-140 of the EB-2, with I-140 downgrade application along the I-485 application at the same time, even though the I-140 for EB3 is not approved.