Watch Roland Koo (Offering Manager for IBM Enterprise Products and Compilers on z/OS) explaining why you should migrate to the latest COBOL Version 6.
Find more resources and tips on migration process at Migration Portal https://www.ibm.com/support/knowledgecenter/SS6SG3/migration-portal.html
Try out Automatic Binary Optimizer for z/OS (ABO) at no charge for 90 day! https://www.ibm.com/products/automatic-binary-optimizer-zos/details