Nouns

(n)replacement, replacingthe act of furnishing an equivalent person or thing in the place of another“replacing the star will not be easy”
(n)surrogate, alternate, replacementsomeone who takes the place of another person
(n)substitution, permutation, transposition, replacement, switchan event in which one thing is substituted for another“the replacement of lost blood by a transfusion of donor blood”
(n)substitute, replacementa person or thing that takes or can take the place of another
(n)refilling, replenishment, replacement, renewalfilling again by supplying what has been used up
(n)successor, replacementa person who follows next in order“he was President Lincoln's successor”