Existing CICS® VT application objects, such as your DIM and DDM drivers modules, are compatible between version 1.2 and version 2.1. Drivers generated with version 1.2 will run in a version 2.1 system. Drivers generated with version 2.1 will run in a version 1.2 system. Version 2.1 uses the version 1.2 mapping tables.
There are no required changes to your existing batch JCL. However, if the DD statement for your migrated VSAM files includes a DSN= parameter, new version 2.1 function is invoked automatically. This is explained in Upgrading the batch subsystem.
Version 1.2 FBE and IRD exits are compatible and no changes or recompiles are required for version 2.1.
There is no advantage in maintaining separate version 1.2 and version 2.1 systems. The recommendation is to use version 2.1 as soon as you have completed the appropriate upgrade tasks.