When you change your calorie goals, it will update your diary from the current day moving forward. Your previous goals should not be impacted from these changes. This is also true when MyFitnessPal automatically updates your goal based on your weight loss. There is also not a way to change any past goals to show a different goal than when they were set at that time, but we will definitely look into making this area editable for a future possibility as well.
For more information on updating your goals, please see this article here.