necessary to load data for newly enhanced fields only for data source

Hi all,

            I am having cube related to SD, we have enhanced a data source with three fields, now we want to load only newly added three fields only, since already cube containing historical data almost 1 year data is there, we are not able to delete old data and reload it again. now i want to load data for that three fields also for past data and so on..

so please let me know what necessary action need to take.