die Ablösung
replacement
The process in which one person replaces another in a position, office, or task.
Nach der Wahl stand die Ablösung des alten Bürgermeisters fest.
After the election, the replacement of the old mayor was certain.

replacement / detachment / payoff
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replacement
The process in which one person replaces another in a position, office, or task.
Nach der Wahl stand die Ablösung des alten Bürgermeisters fest.
After the election, the replacement of the old mayor was certain.

'Die Ablösung' (replacement / detachment / payoff) is the noun corresponding to the verb 'ablösen'. The verb 'lösen' (to loosen / solve), combined with the prefix 'ab-' (away, down), can mean 'to replace', 'to come loose', or 'to repay', depending on the context. The noun describes the process in each case.
Der neue Mitarbeiter wird seinen Kollegen in zwei Wochen ablösen.
The new employee will replace his colleague in two weeks.
The thematic context makes the meaning clear. In politics or work, it is about replacing a person. In medicine or technology, it is about physical separation. In finance, it is about the repayment of debts.
Die Ablösung des alten Trainers wurde in der Presse diskutiert.
The replacement of the old coach was discussed in the press.
To express who or what is being replaced, you usually use the genitive or the preposition 'von' + dative. Both are correct and common.
Die Ablösung des alten Systems verlief reibungslos.
The replacement of the old system went smoothly.
In a medical context, the combination 'Netzhautablösung' (retinal detachment) is a fixed term for a serious eye condition. Here the word is often used as a compound.
Eine Netzhautablösung muss umgehend behandelt werden.
A retinal detachment must be treated immediately.
In a financial context, 'Ablösung' (payoff) is typically combined with types of debt. People often speak of the 'Ablösung eines Kredits' (payoff of a loan), 'eines Darlehens' (of a loan), or 'einer Hypothek' (of a mortgage).
Die Bank informierte ihn über die Konditionen für die Ablösung seiner Hypothek.
The bank informed him about the terms for the payoff of his mortgage.
change / switch
'der Wechsel' and 'die Ablösung' both have to do with replacement. 'Wechsel' is the more general word and simply describes something new taking the place of something old. 'Ablösung' is used especially when one person replaces another in a task, position, or office. The words can be confused because in both cases, someone or something does not remain. But when it is specifically about one person taking over from another, 'Ablösung' is more precise. When the type of change is more general, 'Wechsel' fits better.

Um sechs Uhr kam die Ablösung, und der Wachmann konnte nach Hause gehen.
At six o'clock, the replacement arrived, and the security guard could go home.
Der Wechsel zu einem neuen Arbeitsplan hat im Büro viele Fragen ausgelöst.
The switch to a new work schedule raised many questions in the office.