Will FCC EMC tests need to be redone?
This is certainly not the place to ask for legal or regulatory advice, but my quick 5 minutes on Google suggests that it is only future certifications to be performed by the new smaller set of labs. Existing certification is still verified (on an ongoing basis) by market purchases, so should not be subject to any lower standard today or in future. A fraudulent test report is still no defence for shipping non-compliant product.

Where there could be problems is for product intending to reach compliance before the cut-off, but failing to make the deadline for some reason. In that case you'd need to repeat the testing.

Assuming the rules are for honest reasons, I guess the change is because the number of different products is increasing, and market sampling is becoming less effective at catching incorrectly certified products.