You must only execute this activity if you have changed the stock check indicator in the serial number settings and goods movements were already performed with serial numbers.
Since the indicator, as to whether stocks should be checked for individual stocks, is copied from the presettings in the data records, for this to be subsequently changed, the existing serial numbers must be adjusted to the changed presettings.
These adjustments are made using a report which you can start here. Execute program 'RISERNR9'
The runtime can be long depending on the number of data records.
Note the report documentation