Source migration is not required for most programs and can occur after you have moved your OS/VS COBOL or VS COBOL II programs to run with Language Environment.
Source migration usually consists of upgrading the source language level that is used (such as from the COBOL 74 Standard supported by OS/VS COBOL to the COBOL 85 Standard supported by Enterprise COBOL). Source migration is also required in a few instances to enable your applications to run under Language Environment.
Many conversion tools exist to aid in upgrading your source code. For details, see Conversion tools for source programs.