A word for a temporary change of employment duties, but no change in position

You may have heard “seconded.” The accent here is on the second syllable. The employee keeps his position in the original organization, with benefits, pay, etc., but is assigned (“seconded”) to another organization or unit for a period of time and reports to them; that unit likely reimburses the original organization for the employee’s pay etc.

A law firm sometimes seconds a lawyer to a major client for the experience and mutual benefits.

Originally military; more common in England than the US. See https://www.merriam-webster.com/dictionary/second

One of the latter definitions, as this is not the most common use of the word.