Is there any impact on open PO lines if we don't maintain BPA start date?

Summary:

We are about to implement the "Specify a Start Date for Blanket Purchase Agreement Lines" feature. We want to understand if there will be any impact to all open PO lines if we do not maintain the start date in existing BPA lines (meaning we leave it blank, like it currently is).

Additionally, is it mandatory to maintain a start date for each of the existing BPA lines if we adopt this feature? Or we can still leave it blank and decide to update them phase by phase later?