WordNet sense 1 (someone who takes the place of another person):

replacement, surrogate

WordNet sense 2 (allowing a choice):

alternative

WordNet sense 3 (occurring by turns; first one and then the other):

alternating

WordNet sense 4 (recurring in cycles):

cyclic, cyclical

WordNet sense 5 (be an understudy or alternate for a role):

understudy

WordNet sense 6 (reverse (a direction, attitude, or course of action)):

flip, flip-flop, interchange, switch, tack

WordNet sense 7 (go back and forth; swing back and forth between two states or conditions):

jump

WordNet sense 8 (do something in turns):

take turns