When putting away or stocking recently dated freightwhat should be done with the older existing freight that is already there?

It depends on your racking system.

If you are running your warehouse properly, every piece of freight that goes in is dated before you put it in the racking. When you pull this merchandise to sell, you pick the oldest pallet. Some places like to move freight around so the oldest pallets are on the lowest tiers, but that's unnecessary work.