PGS/PGD on already frozen embryos

PGS/PGD on already frozen embryos

We have 4 frozen embryos that weren't tested. After two losses I have zero desire to transfer any of them without testing them. We just emailed our RE asking if it was possible to thaw them and test them, and he said it is but not without putting the embryos at risk. Ours weren't frozen until day 6. I think we're going to set up an appt with him to get more info, but has anyone done this? Not sure what he meant about risk.