Chris,
Unfortunately, you are not able to change a custom field to add a formula, this has to be configured when the custom field is first created as the Field Type and Formula settings are locked and cannot be changed. you will need to create a new field with formula enabled.
As for the find and replace, this would need to be a workflow rule or Custom Action to allow you to make the change. A workflow rule can be created to basically copy the data from original field to the new custom field.