backup man, fill-in, reliever, stand-in, backup, substitute, relief - someone who takes the place of another (as when things get dangerous or difficult); "the star had a stand-in for dangerous scenes"; "we need extra employees for summer fill-ins"

vary, alter, change - make or become different in some particular way, without permanently losing one's or its former characteristics or essence; "her mood changes in accordance with the weather"; "The supermarket's selection of vegetables varies according to the season"

exchange - hand over one and receive another, approximately equivalent; "exchange prisoners"; "exchange employees between branches of the company"

rotate - exchange on a regular basis; "We rotate the lead soprano every night"

memorise, memorize, con, learn - commit to memory; learn by heart; "Have you memorized your lines for the play yet?"

change by reversal, reverse, turn - change to the contrary; "The trend was reversed"; "the tides turned against him"; "public opinion turned when it was revealed that the president had an affair with a White House intern"

act, move - perform an action, or work out or perform (an action); "think before you act"; "We must move quickly"; "The governor should act on the new energy bill"; "The nanny acted quickly by grabbing the toddler and covering him with a wet towel"

cyclical, cyclic - recurring in cycles

secondary - of second rank or importance or value; not direct or immediate; "the stone will be hauled to a secondary crusher"; "a secondary source"; "a secondary issue"; "secondary streams"

cyclical, cyclic - recurring in cycles

botany, phytology - the branch of biology that studies plants