Unable to change the Date storage property for top parent members of custom dimensions.

Aggregation is not taking place in case of top parent members (Total Cost Center, All Cost Center, Total Location, All Location) of custom dimensions Cost center and Location respectively, as the "store" data storage property is displaying for these members. We want to change the data storage property to "Dynamic Calc" but unable to do so as it is greyed out. kindly help us resolving this issue so that aggregation can be achieved at Parent members.