substituting

(noun) working as a substitute for someone who is ill or on leave of absence. Synonyms: subbing.

(verb) put in the place of another; switch seemingly equivalent items. Synonyms: substitute, replace, interchange, exchange. Examples: "The con artist replaced the original with a fake rembrandt." "Substitute regular milk with fat-free milk." "Synonyms can be interchanged without a changing the context's meaning."

(verb) be a substitute. Synonyms: substitute, sub, stand in, fill in. Examples: "The young teacher had to substitute for the sick colleague." "The skim milk substitutes for cream--we are on a strict diet."

(verb) act as a substitute. Synonyms: substitute, deputize, deputise, step in. Example: "She stood in for the soprano who suffered from a cold."