to do in place of or on behalf of (someone)
 to change places;  to relieve one another;  to replace someone
 to alternate;  to take turns
 to supplant;  to supersede;  to replace;  to take the place of
 to change completely;  to switch (to);  to shift (to another mode)
 to alternate;  to take turns
 to change places;  to relieve (one another)