The relevant question here is, what does "change the bound spell" mean exactly.
From Merriam Webster's online Dictionary:
Full Definition of changechanged changing transitive verb
- a : to make different in some particular : alter <never bothered to change the will>
b : to make radically different : transform <can't change human nature>
c : to give a different position, course, or direction to
- a : to replace with another <let's change the subject>
b : to make a shift from one to another : switch <always changes sides in an argument>
c : to exchange for an equivalent sum of money (as in smaller denominations or in a foreign currency) <change a 20-dollar bill>
d : to undergo a modification of <foliage changing color> e : to put fresh clothes or covering on <change a bed>
It's clearly used as a transitive verb here so I left out the meanings for the intransitive case.
So in our context 2a is the meaning that fits best (according to the example sentence)
So "to change the bound spell" means "to replace the bound spell with another". So clearly two cards are involved here and one is replaced by another. You cannot replace a card with itself. So you may not remove the bound spell and then add it again. But you may replace the spell with another copy of the same spell. This would not violate the "never attach two copies of the same spell to the same object"-rule, because one spell replaces the other and there is no time where both are attached simultaneously regardless of the order in which you exchange them.