Create a project method and copy and paste the following code into the method, modifying the table and field name to the appropriate table and field name. After this is done, in the user environment, click "Special" on the menu bar, and select "Execute Method".

C_LONGINT($i) ` myNewUniqueIDField should also be Long Integer
ALL RECORDS([myTable]) ` grab all the records that exist
For ($i;1;Records in selection([myTable])) ` loop through all the records
[myTable]myNewUniqueIDField:=$i ` assign a new unique longint identifier
SAVE RECORD([myTable]) ` save the change
NEXT RECORD([myTable]) ` move to the next record
End for ` lather, rinse, repeat