I successfully moved my Crocea last year, including cutting the byssal threads in the tank (the rock he was attached to was way too big to move). I got the longest utility knife blade I could find at Home Depot. I elevated the clam a little and kind of sawed away underneath. Bingo. I think the important mistake to avoid is just not to rip the clam off the rock, but to cut the threads instead.
Now my clam is in my other tank, happier under halides, and has reattached to a rock.