My oak experience isn't with burr, but they have relatively fragile taproots at that stage of development. I have successfully extracted some from a similar situation by sssssllllloooowwwwwwllllyyyyyy submerging the pot in a 5 gallon bucket until the potting mix and saplings floated themselves out of the pot, allowing me to separate them.
@LibertyLizard might be able to offer better advice than I can, they've been doing some oak transplanting as well.