The product (item) labels are not associated with a specific shipment - just for the items. You won’t need to replace those.
The carton labels are different (SPD is n of x, where LTL are just numbered). If you do not change to LTL, the carrier can not book a delivery appointment.
If you have not “Completed and Shipped” your shipment, you can change from SPD to LTL and should not need to create a new shipment. If you did “Complete”, never do that until the shipment is on the way to final destination.
Amazon will NOT deal with carton labels at all.
I suggest that when your shipment gets palletized, you have them use the proper labels and just cover the old ones.