Do you want the data in the field you want to change into a summary field? If so, define a new field as your summary field. If you don't want the data that is currently stored in the field, then you can change it into a summary field--discarding the data now stored in the field.
Summary fields can be thought of as special calculation fields that add up ( or perform other operations on ) values across all the records in your current found set, or, in certain situations, in a specific, sorted subset of your found set. This should be a separate field from the value being summarized.
Aha! Thanks for that. It's easy when you know how! :)