Hi Pinky - If you overwrite the workbook while uploading to server, they do not have to subscribe it again. All the subscriptions to that workbook will stay in tact. In case you delete workbook and upload again with same name, you will lose subscriptions. Hope it helps you.
Once we re published same name then subscription will be cancelled.
Thanks everyone. It looks like if you overwrite a workbook which has sources connected from the server, subscriptions remian intact but if the datasources are locally saved, the subscriptions get lost.