When I move clams attached to rock I always do so by chipping out a small patch of the rock from under their foot. That is the only sure way to not hurt the clam.
This is undoubtedly going to catch some flak as being cruel to the critter, but in the past I have used a very new, sharp razor blade and cut the thread as close to the rock as possible. Clams have never had any problems and usually reattach to their new substrate within a day or two.
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