Η «αφαίρεση» είναι removal (κατάλληλο για το συγκεκριμένο κείμενο) ή subtraction. Εδώ θα ταίριαζε να προδώσουμε το κείμενο και να βάλουμε «αντικατάσταση», substitution.

Προσοχή θέλει και ο τρόπος που θα διατυπώσουμε το ρήμα «αφαιρώ» στη συνέχεια. «Αφαιρώ» είναι remove (καλύτερο εδώ) και take away, subtract κ.λπ. Τα substitute, replace, swap είναι «αντικαθιστώ» και θέλουν προσοχή στη διαφορετική σύνταξη.

If one removes a part of one’s body and adds another in its place / replaces it with another, ….
However, if one removes one’s memories or convictions and replaces them with new ones, ….

If one replaces a part of one’s body with another, …
However, if one replaces one’s memories or convictions with new ones, …

If one swaps a part of one’s body with another, ….
However, if one swaps one’s memories or convictions with new ones, …

Προτείνω να μη χρησιμοποιηθεί το ρήμα substitute επειδή εύκολα μπορεί να γίνει λάθος. Αντιγράφω από το επίτομο λεξικό της Οξφόρφης (ODE):

Traditionally, the verb substitute is followed by for and means ‘put someone or something in place of another’, as in she substituted the fake vase for the real one. From the late 17th century substitute has also been used to mean ‘replace someone or something with something else’, as in she substituted the real vase with the fake one. This can be confusing, since the two sentences shown above mean the same thing, yet the object of the verb and the object of the preposition have swapped positions. Despite the potential confusion, the second, newer use is well established, especially in some scientific contexts and in sport (the top scorer was substituted with almost half an hour still to play), and is now generally regarded as part of normal standard English
https://en.oxforddictionaries.com/definition/substitute