You are considering archiving or deleting old programs, and you are wondering how that will affect the Lead field: Acquisition Program.
If you delete a Marketo Program, the field will append a "deleted" at the end of the Program Name.
For example, if the Program Name was "July 2016 newsletter" for Acquisition Program, after deleting the Program, the field value would change to: "July 2016 newsletter - deleted (####)". The numbers represent an activity ID on the backend.
Note: Archiving Programs does not affect the Acquisition Program.