Remember previous value of the field

Remembering until when? During the update and the save operation (Templates, OnSaving-Script, Process gen.conditions, ...) you can access the old-value using the $-notation.

$UID_Department[o]$

You can enable History Features (Common\ProcessState\PropertyLog) without an additional database.

However, the values are only stored for a period of <LifeTime>, regardless of how often they have been changed.

If you only need the previous value for a single field at any time, you might want to save the old value to a separate field when it gets updated (in the column template or format script).