Alt-text should always be added and never changed. The same rules apply here. If you want to change the alt-text, upload the photo afresh. Alt-text should contain contextual information relating to your story, as well as, your chosen keyword or keyphrase. Very very important in obtaining SEO excellence.
For the “stock photos” it is fine to use generic descriptions of what the image is about. For news, however, the context can be specific to the event or the person. Your alt-text should always contain your keyword/keyphrase.