Formula fields are types of fields that calculate a value based on an expression or formula that references other fields or constants. Formula fields prevent administrators from changing their field type once they are created because they may be referenced by other functionalities such as reports, validation rules, workflow rules, etc., that depend on their data type and value. If a formula field is referenced by other functionalities, then changing its field type may cause errors or unexpected results. Reference: https://help.salesforce.com/s/articleView?id=sf.fields_about_formulas.htm&type=5