I believe that the relocation of the adductors takes place simply by growth.  New muscle fibers would grow predominantly towards the growing edges of the shell.  The shell area underlying the muscle is a distinctive structure known as myostracum.  Even in otherwise entirely calcitic bivalves, this is aragonite.  It does not detach the muscle and reattach it, as the trace of the myostracum inside the shell is continuous.  I do not know biochemical details of the attachment, however.

Dr. David Campbell
    "Old Seashells"

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    Saint Mary's College of Maryland
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    "He had discovered an unknown bivalve, forming a new genus"-E. A. Poe, The Gold Bug