There's a possibility to keep a history per org. unit. By means of the "Effective date", you can create a new version of the org. unit. The settings in that version are applicable as from that particular date.

That date is also taken into account when selecting a (new) manager for the org. unit.

On that effective date, the employee (who needs to be selected as manager) needs to be active. If not, he won't be shown in the search results. After all, it's impossible for an employee to be manager of the org. unit since the 1st of January, if he only starts with the company on the 1st of July of that year.

How to resolve this? By creating a new version of the org. unit through "Edit in time". See the full helpcenter article for more info.