Entering the Serial Number Profile in the Material Master Record 
Prerequisites
The serial number profile must be defined in Customizing for Plant Maintenance.
Procedure
As long as no serial numbers exist for the material you are able to cancel or change the serial number profile.
As soon as serial numbers exist for the material and you want to cancel or change the serial number profile, you should note the following:

If backflushing is defined for the material in the master record, you cannot enter a serial number profile that has serial number requirement for goods movements (see procedure MMSL in Serial Numbers in Inventory Management).
You can also create serial number master records if no serial number profile is entered in the material master record. However, use in business procedures is only possible if the serial number profile has been entered in the material master record.