Change
Substitution, Shift
noun, する verb
If you mix and substitute two people, that's a change of personnel, or a substitution. Perhaps those people are even working in shifts, so they mix and substitute each other at regular intervals.
You'll see this word in pretty diverse contexts, because it can refer to anything from a change of political regime, to the substitution of a player during a match, to factory shifts. The underlying meaning is that one person or group is switched out with another.
こうたい
This is a jukugo word that uses the on'yomi readings of the kanji. You haven't learned this reading for 代 yet, so here's a mnemonic to help you:
A change calls for a tie (たい). If you're the new company president, or you're going in to work to start your shift, you'd better make sure you look the part, so get that tie nice and straight.
十六年ぶりの社長交代です。
This is the first change of company president in sixteen years.
ラグビーなど、多くのスポーツで交代のルールがあります。
In many sports, such as rugby, there are rules for substitution.
交代の時に来なかったし、電話をかけても出ないんだ。
He didn't show up for his shift, and he doesn't answer his phone when I call him.