If the file size is incresed in Cobol program it has to get reflect in RUN JCL.
1.Change the Record length in File section to current requirement length.
2.Present in Production if the code is restricted to write 3500 records then change the Perform Until para (for Input file, they might be count used in the Until para)
count from 3500 to current requirement.
Ex: Perform Infile-read para until wk-count = 5000 or Infile-Eof.
here i assumed the count to 5000.
Increment the wk-count when the record was written to output file.