ISBN is a custom column as you mention. Changing the lookup name (#aname) should not affect the contents by itself. OTOH if you created this from other column with a formula, the OLD lookup name might be part of the formula.
OTOH if you changed anything else other than names
(I did not think it allowed that)
ISBN is just one of the possible values in {identifiers} (a 2 part value field. Identifier_type:value. this field accept multiple, all different, types)