The core of the language should be almost the same. It’s not going to that far off. For example, say the last time you coded in COBOL was back in 2000 and now 19 years later try the same code. The code may work but it may not. What is more important is It comes down to what release you are working on as to if any commands have been dropped or enhanced. The interface for entering the code may vary as well. Without more specifics as to the two different platforms , the mainframe and the i-series, it hard to name specific changes.