Subtle and not so Subtle Differences

Some action can be required for existing applications:

Some OS/VS COBOL applications can be executed without change. However, it is necessary to recompile the COBOL parts for a mixed COBOL-PL/I application.
SCEESAMP contains link-edit JCL to remove older versions of library modules so that the application can be relinked with LE. This may resolve some problems. It is necessary to do this for programs compiled with older versions of the PL/I compiler or for a mixed PL/I-FORTRAN application.
Important note: There is a problem with older versions ofthe PL/I DATE builtin function. You can find more information at this page.
Refer to this page for more information about migrating to Language Environment.

Some differences that might not be obvious include:

LE does not manage dynamic storage in the same way as earlier runtime environments. Coding problems such as failure to initialize variables may surface when an application is relinked with LE. This can often be circumvented using the STORAGE run-time option. There is some overhead associated with this circumvention.
Initialization problems related to increased virtual storage use. Simply increasing the REGION will make this type of problem go away.
Because LE combines run-time services for compiler products which previously had their own separate run-time libraries, some choices had to be made. For example, the message file output is provided through common LE services. This may result in a different DDNAME being used for some compiler products.
Some problems may surface in mixed language environments. For example, in a mixed C and PL/I environment, some conditions that were previously suppressed in hardware may be handled via software error recovery.
Choose the default settings for run-time parameters carefully.