The formula for creating a visual indicator flag on each case based on the case priority should use the IMAGE and CASE functions. The IMAGE function returns an image for a given URL, and the CASE function evaluates an expression and returns a value based on that expression. Option A and D use these functions correctly, while option B and C do not.

Changing the data type of a field used in an apex class from number to text will require the app builder to perform additional steps in order to retain existing functionalities. This is because changing the data type of a field may affect the apex code that references or manipulates the field value, and may cause compilation errors or unexpected results. The app builder will need to update the apex code to handle the new data type and ensure that the logic and calculations are still correct. Option B is incorrect because changing the data type of a field used in a report from a text to an encrypted field will not require additional steps, as encrypted fields can still be used in reports with some limitations. Option C is incorrect because changing the data type of a field used as an external id from number to text will not require additional steps, as external id fields can be either number or text. Option D is incorrect because changing the data type of a field used in lead conversion from number to text will not require additional steps, as lead conversion fields can be mapped regardless of their data types.