Miles only Reward Sale flight . Change of Date help required

Hi there.
Can you please help me regarding this question. If I book a Full Miles flight in the reward sale therefore saving 30% miles. eg Flight to NYC 24,500 each way plus applicable taxes. If at a later date (after the reward sale, say next month) I wish to change to dates of my flight, do I simply pay £30 to change the date if they have availability, or do they also charge me the extra miles and treat it as a new booking ? If the latter was the case then I wouldn't actually have enough miles !! Thanks for any help. KR .
Can you please help me regarding this question. If I book a Full Miles flight in the reward sale therefore saving 30% miles. eg Flight to NYC 24,500 each way plus applicable taxes. If at a later date (after the reward sale, say next month) I wish to change to dates of my flight, do I simply pay £30 to change the date if they have availability, or do they also charge me the extra miles and treat it as a new booking ? If the latter was the case then I wouldn't actually have enough miles !! Thanks for any help. KR .